Laminated Separator, Polyolefin Microporous Membrane, and Separator for Electricity Storage Device

ABSTRACT

Disclosed is a laminated separator including a first polyolefin microporous layer and a second polyolefin microporous layer which is laminated on the first polyolefin microporous layer and which is different from the first polyolefin microporous layer, wherein at least one of the first microporous layer and the second microporous layer includes an inorganic particle having a primary particle size of 1 nm or more and 80 nm or less.

TECHNICAL FIELD

The present invention relates to a laminated separator, a polyolefinmicroporous membrane, a separator for an electricity storage device andthe like.

BACKGROUND ART

Polyolefin resin microporous membranes have been used as separators forbatteries, in particular, separators for lithium ion batteries. Lithiumion batteries have been used in small size electronic devices such ascellular phones and notebook-size personal computers, and also have beenattempted to be applied to electric tools, hybrid vehicles, electricvehicles and the like.

For separators for lithium ion batteries, polyethylene microporousmembranes have hitherto been used. This is because polyethylenemicroporous membranes are excellent in permeability, and have a functionto perform shutdown of the current by blocking continuous pores throughmelting the polymer at 130° C. to 150° C., for the purpose of ensuringthe safety of the batteries. The term “shutdown” means a phenomenon inwhich the pores of a microporous membrane are blocked by a molten resinto increase the electrical resistance of the membrane and consequentlythe membrane shuts down the flow of the lithium ions.

In this connection, from the viewpoint of more improving the safety ofelectricity storage devices, the separator is required to have, inaddition to mechanical properties, above a certain level, not to bebroken during repeated charge-discharge cycles, the properties such asthe property (fuse property) to rapidly halt the battery reaction whenabnormal heating occurs, and the property (short-circuit property) toprevent a dangerous situation of the direct reaction between thepositive electrode material and the negative electrode material throughmaintaining the shape of the separator even when the temperature comesto be high.

It is recognized that the lower is the temperature at which the fuseoccurs, the higher is the effect to safety. The higher temperature atwhich short-circuit occurs is preferable from the viewpoint ofmaintaining the film shape even after the blocking of the pores andmaintaining the insulation between the electrodes.

Recently, for the purpose of further improving the safety of batteries,there have been proposed a method in which a layer mainly composed of aninsulating inorganic filler is formed between the separator and each ofthe electrodes (for example, Patent Literature 1), and a separator madeof a polyethylene microporous membrane including an inorganic substance(for example, Patent Literature 2 and Patent Literature 3).

As an attempt to increase the heat resistance of the separator, therehave been performed an attempt to blend polypropylene high in meltingpoint with polyethylene, and an attempt to laminate a polyethylenemicroporous membrane and a polypropylene microporous membrane (see, forexample, Patent Literature 4 and Patent Literature 5).

CITATION LIST Patent Literature Patent Literature 1: Japanese PatentLaid-Open No. 11-080395 Patent Literature 2: Japanese Patent No. 4049416Patent Literature 3: Japanese Patent Laid-Open No. 2001-266828 PatentLiterature 4: Japanese Patent Laid-Open No. 63-243146 Patent Literature5: Japanese Patent Laid-Open No. 62-010857 SUMMARY OF INVENTIONTechnical Problem

However, any of the separators described in Patent Literature 1 toPatent Literature 3 still has room for improvement from the viewpoint ofbringing about a high level of compatibility between the heatresistance, the shutdown property and the cycle property.

In view of such circumstances, a first problem to be solved by thepresent invention is to provide a separator having a high level ofcompatibility between the heat resistance, the shutdown property and thecycle property.

Any of the microporous membranes described in Patent Literature 4 andPatent Literature 5 still has room for improvement from the viewpoint ofthe property (cycle property) of satisfactorily maintaining the batterycapacity after repeated charge-discharge cycles.

In view of such circumstances, a second problem to be solved by thepresent invention is to provide a polyolefin microporous membranesuitable as a separator capable of improving the cycle property of anelectricity storage device.

Solution to Problem

In view of the aforementioned circumstances, the present inventors madea diligent study and consequently have found that a laminated separatorincluding inorganic particles having a particle size falling within aspecific range and having a laminated structure is capable of having ahigh level of compatibility between the heat resistance, the shutdownproperty and the cycle property.

The present inventors have also found that a polypropylene-basedpolyolefin microporous membrane having a specific composition is capableof solving the aforementioned second problem.

Further, the present inventors have found that the aforementioned secondproblem can be solved by mixing a specific propylene copolymer in aspecific amount in the polypropylene based microporous membrane.

Further, the present inventors have found that the aforementioned secondproblem can be solved by mixing a specific propylene copolymer in aspecific amount in the polypropylene based microporous membraneincluding an inorganic filler.

Specifically, the present invention is as follows.

[1] A laminated separator including a first polyolefin microporous layerand a second polyolefin microporous layer which is laminated on thefirst polyolefin microporous layer and which is different from the firstpolyolefin microporous layer, wherein at least one of the firstmicroporous layer and the second microporous layer includes an inorganicparticle having a primary particle size of 1 nm or more and 80 nm orless.

[2] The laminated separator according to [1], wherein the differencebetween the concentration C1 of the inorganic particle in the totalamount of the polyolefin resin and the inorganic particle in the firstpolyolefin microporous layer and the concentration C2 of the inorganicparticle in the total amount of the polyolefin resin and the inorganicparticle in the second polyolefin microporous layer is 10% by mass ormore and 95% by mass or less.

[3] The laminated separator according to [1] or [2], wherein theinorganic particle is one or two or more selected from the groupconsisting of silicon oxide, aluminum oxide and titanium oxide.

[4] The laminated separator according to any one of [1] to [3], wherein:the laminated separator has a two-type three-layer structure includingthe first polyolefin microporous layers as surface layers and the secondpolyolefin microporous layer as an intermediate layer; and the firstpolyolefin microporous layer includes polyolefin resin in an amount of 5to 90% by mass based on the total amount of the polyolefin resin and theinorganic particle which are the constituent components of the firstmicroporous layer, and the second polyolefin microporous layer includespolyolefin resin in an amount of 60 to 100% by mass based on the totalamount of the polyolefin resin and the inorganic particle which are theconstituent components of the second microporous layer.

[5] The laminated separator according to [4], wherein in each of thesurface layers, the polyolefin resin includes polyethylene andpolypropylene, and at the same time, the proportion of the polypropylenein the total amount of the polyethylene and polypropylene is 10% by massor more and 95% by mass or less.

[6] A polyolefin microporous membrane formed of a polyolefin resin, as amain component, including 50 to 99% by mass of polypropylene and 1 to50% by mass of propylene-α-olefin copolymer, wherein the content of theα-olefin in the propylene-α-olefin copolymer is more than 1% by mass and15% by mass or less.

[7] The polyolefin microporous membrane according to [6], wherein themixing ratio (polypropylene/propylene-α-olefin copolymer) (mass ratio)between the polypropylene and the propylene-α-olefin copolymer is 1.5 ormore and 60 or less.

[8] The polyolefin microporous membrane according to [6] or [7], whereinthe polyolefin resin further includes a high-density polyethylene, andthe proportion of the high-density polyethylene in the polyolefin resinis 5 to 45% by mass.

[9] The polyolefin microporous membrane according to any one of [6] to[8], wherein the polyolefin resin further includes an inorganic filler,and the proportion of the inorganic filler in the total amount of theinorganic filler and the polyolefin resin is 1 to 80% by mass.

[10] The polyolefin microporous membrane according to [9], wherein theinorganic filler is one or more selected from the group consisting ofsilica, alumina and titania.

[11] A laminated polyolefin microporous membrane wherein on at least oneside of the polyolefin microporous membrane according to any one of [6]to [10], another polyolefin microporous membrane different from thepolyolefin microporous membrane is laminated.

[12] A separator for an electricity storage device, formed of thepolyolefin microporous membrane according to any one of [6] to [10], orthe laminated polyolefin microporous membrane according to [11].

[13] A polyolefin microporous membrane formed of a propylene-based resincomposition including a polypropylene-based resin having(polypropylene)/(propylene copolymer) (mass ratio) of 80/20 to 0/100 asa main component, wherein the melting point of the propylene copolymeris 120° C. or higher and 145° C. or lower.

[14] The polyolefin microporous membrane according to [13], wherein thepropylene copolymer is a random copolymer.

[15] The polyolefin microporous membrane according to [13] or [14],wherein the content of the comonomer included in the propylene copolymeris more than 1% by mass and 20% by mass or less.

[16] The polyolefin microporous membrane according to any one of [13] to[15], wherein the propylene-based resin composition further includes ahigh-density polyethylene, and the proportion of the high-densitypolyethylene in the propylene-based resin composition is 5 to 50% bymass.

[17] The polyolefin microporous membrane according to any one of [13] to[16], wherein the propylene-based resin composition further includes aninorganic filler, and the proportion of the inorganic filler in thepropylene-based resin composition is 5 to 60% by mass.

[18] A laminated polyolefin microporous membrane, wherein on at leastone side of the polyolefin microporous membrane according to any one of[13] to [17], another polyolefin microporous membrane different from thepolyolefin microporous membrane is laminated.

[19] A separator for a nonaqueous electrolyte including the polyolefinmicroporous membrane according to any one of [13] to [17], or thelaminated polyolefin microporous membrane according to [18].

[20] A method for producing the polyolefin microporous membraneaccording to any one of [13] to [17] or the laminated polyolefinmicroporous membrane according to [18], the method including:

(1) a kneading step of forming, according to an intended layerstructure, a kneaded mixture by kneading a propylene-based resincomposition including a polypropylene-based resin having a ratio of(polypropylene)/(propylene copolymer) (mass ratio) of 80/20 to 0/100 asa main component and a plasticizer, and where necessary, a high-densitypolyethylene and an inorganic filler;

(2) a sheet molding step, following the kneading step, of processing thekneaded mixture into a sheet-like molded body (a monolayer molded bodyor a laminated molded body) by extruding the kneaded mixture into asheet and, where necessary, by laminating the resulting sheets into alaminated body, and cooling and solidifying the resulting monolayer bodyor the resulting laminated body;

(3) a stretching step, following the molding step, of forming astretched product by biaxially stretching the sheet-like molded bodywith an area magnification of 20× or more and 200× or less;

(4) a porous body forming step, following the stretching step, offorming a porous body by extracting the plasticizer from the stretchedproduct; and

(5) a heat treatment step, following the porous body forming step, ofheat treating the porous body at a temperature equal to or lower thanthe melting point of the polyolefin resin and widthwise stretching theporous body.

[21] The method according to [20], wherein the heat of fusion of thepropylene copolymer is 60 J/g or more.

[22] A polyolefin microporous membrane, formed of a polypropylene-basedresin composition including 20 to 95% by mass of a polypropylene-basedresin having (polypropylene)/(propylene copolymer) (mass ratio) of 90/10to 0/100 and 5 to 80% by mass of an inorganic filler,

wherein the melting point of the propylene copolymer is 110° C. to 150°C.; and

the (propylene copolymer)/(inorganic filler) (mass ratio) is 0.1/1 to1.5/1.

[23] The polyolefin microporous membrane according to [22], wherein thepropylene copolymer is a random copolymer.

[24] The polyolefin microporous membrane according to [22] or [23],wherein the content of the comonomer included in the propylene copolymeris more than 1% by mass and 20% by mass or less.

[25] The polyolefin microporous membrane according to any one of [22] to[24],

wherein the propylene-based resin composition further includes ahigh-density polyethylene; and

the proportion of the high-density polyethylene in the propylene-basedresin composition is 5 to 60% by mass.

[26] The polyolefin microporous membrane according to any one of [22] to[25], wherein the inorganic filler is any of silica, alumina andtitania.

[27] The laminated polyolefin microporous membrane, wherein on at leastone side of the polyolefin microporous membrane according to any one of[22] to [26], another polyolefin microporous membrane different from thepolyolefin microporous membrane is laminated.

[28] A separator for a nonaqueous electrolyte including the polyolefinmicroporous membrane according to any one of [22] to [26], or thelaminated polyolefin microporous membrane according to [27].

[29] A method for producing the polyolefin microporous membraneaccording to any one of [22] to [26] or the laminated polyolefinmicroporous membrane according to [27], the method including:

(1) a kneading step of forming, according to an intended layerstructure, a kneaded mixture by kneading a polypropylene-based resincomposition including 20 to 95% by mass of a polypropylene-based resinhaving (polypropylene)/(propylene copolymer) (mass ratio) of 90/10 to0/100 and 5 to 80% by mass of an inorganic filler, and a plasticizer,and where necessary, a high-density polyethylene;

(2) a sheet molding step, following the kneading step, of processing thekneaded mixture into a sheet-like molded body (a monolayer molded bodyor a laminated molded body) by extruding the kneaded mixture into asheet and, where necessary, by laminating the resulting sheet into alaminated body, and cooling and solidifying the resulting monolayer bodyor the resulting laminated body;

(3) a stretching step, following the molding step, of forming astretched product by biaxially stretching the sheet-like molded bodywith an area magnification of 20× or more and 200× or less;

(4) a porous body forming step, following the stretching step, offorming a porous body by extracting the plasticizer from the stretchedproduct; and

(5) a heat treatment step, following the porous body forming step, ofheat treating the porous body at a temperature equal to or lower thanthe melting point of the polyolefin resin and widthwise stretching theporous body.

[30] The method according to [29], wherein the heat of fusion of thepropylene copolymer is 60 J/g or more.

Advantageous Effects of Invention

According to the present invention, a separator having a high level ofcompatibility between the heat resistance, the shutdown property and thecycle property is provided.

Additionally, according to the present invention, a polyolefinmicroporous membrane suitable as a separator capable of improving thecycle property of an electricity storage device is provided.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 shows schematic views illustrating a shutdown temperaturemeasurement apparatus used in Examples.

FIG. 2 shows that the microporous membrane 1 is superposed on the nickelfoil 2A, and longitudinally fixed to the nickel foil 2A with a “Teflon(registered trademark)” tape (the shaded section in the figure).

FIG. 3 shows that a “Teflon (registered trademark)” tape (the shadedsection in the figure) was bonded, and then the nickel foil 2B wasmasked with a 15 mm×10 mm window section left unmasked in the centralsection of the foil 2B.

DESCRIPTION OF EMBODIMENTS

Hereinafter, modes for carrying out the present invention (hereinafter,abbreviated as the “embodiments”) are described in detail. It is to benoted that the present invention is not limited to following Embodimentsand can be embodied in various modified forms within the range of thegist of the present invention.

First Embodiment

A laminated separator of a first embodiment of the present invention(hereinafter, abbreviated as “Embodiment 1”) includes a first polyolefinmicroporous membrane (hereinafter, also referred to as the “firstmicroporous layer”) and a second polyolefin microporous membrane(hereinafter, also referred to as the “second microporous layer”)laminated on the first microporous layer and different from the firstmicroporous layer, wherein at least one of the first microporous layerand the second microporous layer includes an inorganic particle (alsodescribed as the “inorganic filler” as the case may be) having a primaryparticle size of 1 nm or more and 80 nm or less.

The primary particle size of the inorganic particle used in Embodiment 1is 1 nm or more and 80 nm or less. An inorganic particle having aprimary particle size falling within the aforementioned range is able tobe finely dispersed in polyolefin in a satisfactory manner, and hence itis inferred that such an inorganic particle is satisfactorily fusionbonded to polyolefin, to enable drastic improvement the melt tension ofthe fibrillar portion of the three-dimensional network skeletonstructure of the separator.

On the other hand, the laminated separator of Embodiment 1 has alaminated structure, the respective layers are formed under differentconditions, and hence it is inferred that the respective layers aredifferent from each other in the properties such as the pore structure,the melting behavior and the dispersion condition of the inorganicparticle. The adoption of an inorganic particle having a primaryparticle size falling within the aforementioned specific range and theaforementioned laminated structure results in realization of anunexpected effect such that a high level of compatibility between theheat resistance, the shutdown property and the cycle property isachieved.

Although the mechanism for this effect is not clear, it is inferred thatthe presence of such aforementioned differences (presence of theinterface(s)) between the respective layers with respect to the meltingbehavior and the like enables to contribute to the improvement the heatresistance of the separator as a whole, and at the same time, incooperation with the use of an inorganic particle having a primaryparticle size falling within a specific range, the pore structure ofeach of the layers is respectively uniformized in an appropriate manner,such pore structure contributes to the improvement the wettability ofthe electrolyte, and accordingly contributes to the improvement thecycle property and further enables to contribute to the improvement theshutdown property.

Here, the first polyolefin microporous layer and the second polyolefinmicroporous layer are different from each other, and the term“different” as used herein may mean the difference either in rawmaterials or in the structure (specifically, for example, differences inporosity and pore structure).

Examples of the polyolefin resin used in the first polyolefinmicroporous layer or the second polyolefin microporous layer include thepolymers (homopolymers and copolymer, and multistage polymers and thelike) obtained by polymerizing the monomers such as ethylene, propylene,1-butene, 4-methyl-1-pentene, 1-hexene and 1-octene. The polymers can beused each alone or in combinations of two or more thereof.

Examples of the polyolefin resin include: polyethylene (for example,low-density polyethylene, linear low-density polyethylene,medium-density polyethylene, high-density polyethylene (density: 0.942g/cm³ or more) and ultra high molecular weight polyethylene);polypropylene (for example, isotactic polypropylene and atacticpolypropylene); polybutene; ethylene-propylene rubber;propylene-ethylene copolymer; and propylene-α-olefin copolymer. In thepropylene-ethylene copolymer and the propylene-α-olefin copolymer, thepositions of ethylene and α-olefin in the polymer chain are notparticularly limited; either a random copolymer or a block copolymer maybe used. Hereinafter, polyethylene and polypropylene are abbreviated as“PE” and “PP,” respectively, as the case may be.

The viscosity average molecular weight of the polyolefin resin ispreferably 50,000 or more and more preferably 100,000 or more; the upperlimit of the aforementioned molecular weight is preferably 10,000,000 orless and more preferably 3,000,000 or less. It is preferable to set theviscosity average molecular weight at 50,000 or more because suchmolecular weight increases the melt tension at the time of melt molding,accordingly the improvement the moldability can be expected, sufficientintermolecular entanglement can also be expected and a high strengthtends to be attained. On the other hand, it is preferable to set theviscosity average molecular weight at 10,000,000 or less, from theviewpoint of performing uniform melt kneading and from the viewpoint ofimproving the moldability of the sheet, in particular, the thicknessstability of the sheet. In particular, it is preferable to set theviscosity average molecular weight at 3,000,000 or less from theviewpoint of improving the moldability.

From the viewpoint of decreasing the shutdown temperature and improvingthe safety, the polyolefin resin may include a low molecular weightresin having a viscosity average molecular weight of preferably 800 ormore, more preferably 900 or more and furthermore preferably 1000 ormore, and an upper limit of the viscosity average molecular weight of10,000 or less.

Examples of such a low molecular weight resin include polyethylene,polypropylene, polybutene and ethylene-propylene copolymer (inclusive ofelastomer); however, from the viewpoint of the membrane formability andthe viewpoint of performing uniformly shutdown, polyethylene andpolypropylene are more preferable.

The proportion of such a low molecular weight resin in the polyolefinresin is preferably 0.1% by mass or more, more preferably 0.5% by massor more and furthermore preferably 1% by mass or more, and the upperlimit of the aforementioned proportion is preferably 20% by mass orless, more preferably 15% by mass or less and furthermore preferably 10%by mass or less.

For the below described layer including an inorganic particle, from theviewpoint of uniformly dispersing the inorganic particle, a high-densitypolyethylene having a viscosity average molecular weight of 200,000 ormore and 3,000,000 or less and an ultra high molecular weightpolyethylene having a viscosity average molecular weight of 1,000,000 ormore are preferably used each alone or in combination.

Examples of the inorganic particle may include: (1) the oxides ornitrides of the elements such as silicon, aluminum and titanium; and (2)the carbonates or sulfates of the elements such as calcium and barium.It is preferable to use the inorganic particles of (1) or (2), from theviewpoint of achieving a higher level of compatibility between the heatresistance and the cycle property of the obtained separator.

Such inorganic particles are preferably present in a microporous layeras separately dispersed therein or may also be present as dispersed in apartially aggregated form.

The lower limit of the primary particle size of the inorganic particleis 1 nm or more, preferably 6 nm or more and furthermore preferably 10nm or more, and the upper limit of the aforementioned primary particlesize is 80 nm or less, preferably 50 nm or less and furthermorepreferably 30 nm or less.

Various additives can be mixed, where necessary, in the resincomposition (hereinafter also referred to as the “polyolefincomposition”) including the polyolefin resin. Examples of such additivesinclude: a phenolic antioxidant, a phosphorus-based antioxidant and asulfur-based antioxidant; a nucleating agent; metal soaps such ascalcium stearate and zinc stearate; an ultraviolet absorber; a lightstabilizer, an antistatic agent, an antifogging agent and a pigment.

The proportion of such an additive in the polyolefin composition ispreferably 5% by mass or less and more preferably 2% by mass or less,and may also be substantially 0% by mass.

The difference between the concentration C1 of the inorganic particle inthe total amount of the polyolefin resin and the inorganic particle inthe first polyolefin microporous layer and the concentration C2 of theinorganic particle in the total amount of the polyolefin resin and theinorganic particle in the second polyolefin microporous layer ispreferably 10% by mass or more, more preferably 20% by mass or more,furthermore preferably 40% by mass or more and particularly preferably60% by mass or more. On the other hand, the upper limit of theaforementioned difference is preferably 95% by mass or less and morepreferably 80% by mass or less. It is preferable to set theaforementioned concentration difference at 10% by mass or more from theviewpoint of improving the heat resistance and the cycle property. Onthe other hand, it is preferable to set the aforementioned concentrationdifference at 95% by mass or less from the viewpoint of ensuring theshutdown property and the puncture strength.

In Embodiment 1, the “inorganic particle concentration” as referred toherein means the proportion of the inorganic particle in the totalamount of the polyolefin resin and the inorganic particle.

The difference between the volume proportion of the inorganic particlein the first polyolefin microporous layer and the volume proportion ofthe inorganic particle in the second polyolefin microporous layer ispreferably 10% by volume or more, more preferably 15% by volume or more,furthermore preferably 20% by volume or more and particularly preferably60% by volume or more. On the other hand, the upper limit of theaforementioned difference is 95% by volume or less and more preferably80% by volume or less.

Hereinafter, the laminated separator of Embodiment 1 is described with afocus on an aspect having a two-type three-layer structure in which thefirst polyolefin microporous layers are the surface layers and thesecond polyolefin microporous layer is the intermediate layer.

The polyolefin resin concentration of the first microporous layerforming each of the surface layers is preferably 5 to 90% by mass andmore preferably 20 to 80% by mass.

Here, the polyolefin resin forming each of the surface layers preferablyincludes PE and PP. The proportion of the PP in the total amount of PEand PP is 10% by mass or more, more preferably 20% by mass or more,furthermore preferably 40% by mass or more, preferably 95% by mass orless, preferably 90% by mass or less and furthermore preferably 80% bymass or less.

The formation of such surface layers is preferable from the viewpoint ofobtaining a satisfactory heat resistance and from the viewpoint ofimproving the stretchability and thus obtaining a microporous membranehaving a high puncture strength.

In Embodiment 1, the “polyolefin resin concentration” as referred toherein means the proportion of the polyolefin resin in the total amountof the polyolefin resin and the inorganic particle.

The C1 in each of the surface layers is preferably 10% by mass or more,more preferably 20% by mass or more, furthermore preferably 40% by massor more and particularly preferably 60% by mass or more, and the upperlimit of the C1 is preferably 95% by mass or less, more preferably 90%by mass or less and furthermore preferably 80% by mass or less. It ispreferable to set the aforementioned proportion at 10% by mass or morefrom the viewpoint of improving the heat resistance and the cycleproperty. On the other hand, it is preferable to set the aforementionedproportion at 95% by mass or less from the viewpoint of improving themembrane formability at a high stretching magnification and thusimproving the puncture strength of the polyolefin resin microporousmembrane.

The use of the inorganic particle in combination with PE and PP in eachof the surface layers, combined with the setting of the proportions ofthe individual components in the aforementioned ranges is preferablefrom the viewpoint of improving the interaction between the polyolefinresin and the inorganic particle and accordingly achieving a higherlevel of compatibility between the heat resistance and the cycleproperty of the obtained laminated separator.

The volume proportion of the inorganic filler in the first polyolefinmicroporous layer is preferably 10% by volume or more, more preferably15% by volume or more and furthermore preferably 20% by volume or more,and the upper limit of the aforementioned volume proportion ispreferably 95% by volume.

The polyolefin resin concentration of the second microporous layerforming the intermediate layer is preferably 60% by mass or more, morepreferably 80% by mass or more and furthermore preferably 90% by mass ormore, and may also be 100% by mass.

The polyolefin resin used for the intermediate layer preferably includesPE as a main component; however, it is possible to use PP in combinationwith PE within a range not to impair the shutdown property. In theintermediate layer, the proportion of PE in the total amount of PE andPP is preferably 60% by mass or more and more preferably 80% by mass ormore, and may also be 100% by mass.

The C2 in the intermediate layer is preferably 60% by mass or less, morepreferably 40% by mass or less, furthermore preferably 20% by mass andyet furthermore preferably 10% by mass or less, and may also be 0% bymass. The aforementioned proportion set at 60% by mass or less ispreferable from the viewpoint of the shutdown property.

The volume proportion of the inorganic filler in the second polyolefinmicroporous layer is preferably 60% by volume or less, more preferably40% by volume or less and furthermore preferably 20% by volume or lessand yet furthermore preferably 10% by volume or less, and may also be 0%by volume.

The specific gravity of the inorganic filler is preferably 1.0 g/cm³ ormore, more preferably 1.2 g/cm³ or more, furthermore preferably 1.5g/cm³ or more, and the upper limit of the aforementioned specificgravity is preferably 10.0 g/cm³ or less.

From the viewpoint of the heat resistance, the shutdown property and thecycle property, preferable is a configuration having a two-typethree-layer structure in which the first microporous layers are thesurface layers and the second microporous layer is the intermediatelayer, wherein the first microporous layer includes polyolefin resin andan inorganic particle, and the second microporous layer does not includean inorganic particle.

On the other hand, from the viewpoint of the shutdown property and thecycle property and the productivity of the battery manufacture,preferable is a configuration having a two-type three-layer structure inwhich the second microporous layers are the surface layers and the firstmicroporous layer is the intermediate layer, wherein the firstmicroporous layer includes polyolefin resin and an inorganic particle,and the second microporous layer does not include an inorganic particle.

The method for producing the laminated separator of Embodiment 1 is notparticularly limited; however, examples of such a method may include aproduction method including the following steps (1) to (4):

(1) a step of melt-kneading the raw materials (polyolefin composition,and a plasticizer) forming each of a plurality of layers;

(2) a step, following the step (1), of coextruding the kneaded mixturesobtained by melt-kneading to form a laminated sheet, and cooling andsolidifying the resulting laminated sheet;

(3) a step, following the step (2), of stretching the laminated sheet inat least one axial direction, with an area magnification of 20× or moreand less than 200×;

(4) a step, preceding or following the step (3), of extracting theplasticizer.

The plasticizer used in the step (1) is preferably a nonvolatile solventcapable of forming a uniform molten resin at a temperature equal to orhigher than the melting point of polyolefin resin when the plasticizeris mixed with polyolefin resin, and is preferably liquid at normaltemperature. Examples of the plasticizer include: hydrocarbons such asliquid paraffin and paraffin wax; esters such as dioctyl phthalate anddibutyl phthalate; and higher alcohols such as oleyl alcohol and stearylalcohol.

The mixing proportion of the plasticizer based on the polyolefincomposition is such that the mixing proportion allows uniform meltkneading, is a proportion sufficient for forming a sheet-likemicroporous membrane precursor and is of an order not to impair theproductivity. Specifically, the content of the plasticizer (plasticizerproportion) in the total amount of the polyolefin composition and theplasticizer is preferably 20% by mass or more and 80% by mass or lessand more preferably 30% by mass or more and 70% by mass or less. Thecontent of the plasticizer set at 80% by mass or less is preferable fromthe viewpoint of maintaining the high melt tension at the time of meltmolding and thus ensuring the moldability. On the other hand, thecontent of the plasticizer set at 20% by mass or more is preferable fromthe viewpoint of ensuring the moldability and the viewpoint ofefficiently stretching the lamellar crystal in the crystalline region ofthe polyolefin resin. The efficient stretching of the lamellar crystalmeans the efficient stretching of the polyolefin resin chain withoutbreaking the polyolefin resin chain, and is probably capable ofcontributing to the formation of uniform and fine pore structure and theimprove of the microporous membrane strength.

The method for melt kneading the polyolefin composition and theplasticizer is preferably such that two or more extruders are used, thepolyolefin resin and, where necessary, the inorganic particle andindividual additives are placed, the plasticizer is introduced at anoptional proportion into the polyolefin composition while the polyolefincomposition is being heated and melted, and the composition is furtherkneaded to yield a uniform molten resin. Additionally, the method formelt kneading the polyolefin resin, the inorganic particle and theplasticizer is preferably such that in a preliminary step of beforehandkneading the polyolefin resin, the inorganic particle and theplasticizer in a predetermined ratio with a mixer such as Henschelmixer, and then the kneaded mixture is placed in an extruder and furtherkneaded with the plasticizer introduced into the kneaded mixture with anoptional proportion while the kneaded mixture is being heated andmelted.

In the coextrusion method in the step (2), preferable is intra-dieadhesion in which molten mixtures from two or more extruders areextruded form a die lip, and it is preferable to use a multiple manifoldmethod or a feed block method. Here, it is preferable to use a flat diesuch as a T-die or a coat hanger die.

The cooling and solidification in the step (2) is preferably performed,for example, with a method in which the product molded into a sheetshape is brought into contact with a heat conductor to cool thesheet-shaped product to a temperature sufficiently lower than thecrystallization temperature of the resin.

In the step (3), as for the stretching direction, at least a uniaxialstretching is adopted. It is preferable to performing ahigh-magnification biaxial stretching since molecular orientation occursin the plane direction to provide a hardly tearable and stablestructure, and a high puncture strength tends to be obtained. Withrespect to the stretching method, simultaneous biaxial stretching,successive biaxial stretching, multiple stage stretching and multipletime stretching may be applied each alone or in all possiblecombinations; however, adopting simultaneous biaxial stretching as thestretching method is particularly preferable from the viewpoint ofincreasing the puncture strength and uniformizing the membranethickness.

The stretching magnification ratio between MD and TD is preferably 0.5or more and 2 or less. The stretching magnification in terms of the areamagnification is preferably in a range of 20× or more and less than200×, more preferably in a range of 20× or more and 100× or less andfurthermore preferably in a range of 25× or more and 50× or less. Whenthe total area magnification is 20× or more, a sufficient puncturestrength tends to be able to be imparted to the membrane, and when thetotal area magnification is less than 200×, preferably the membranefracture tends to be prevented and a high productivity tends to beobtained.

The term “MD” means the lengthwise direction of the separator or thedischarge direction of the raw material resin at the time of membraneformation, and the term “TD” means the widthwise direction of theseparator.

The stretching temperature is preferably equal to or higher than themelting point of polyolefin−50° C. and preferably lower than the meltingpoint of polyolefin. The stretching temperature is more preferably equalto or higher than the melting point of polyolefin−30° C. and equal to orlower than the melting point−2° C., and furthermore preferably equal toor higher than the melting point of polyolefin−15° C. and equal to orlower than the melting point−3° C. The stretching temperature set at atemperature equal to or higher than the melting point of polyolefin−50°C. is preferable from the viewpoint of the high puncture strength. Thestretching temperature set at a temperature lower than the melting pointof polyolefin is preferable from the viewpoint of the reduction of thestretching unevenness.

The step (4) may be of either a batch type or a continuous type;preferably the laminated sheet is immersed in an extraction solvent toextract the plasticizer, then the laminated sheet is sufficiently dried,and thus the plasticizer is substantially removed from the microporousmembrane. For the purpose of suppressing the contraction of thelaminated sheet, it is preferable to constrain the ends of the laminatedsheet during a series of the steps of immersion and drying. The residualamount of the plasticizer in the laminated sheets having been subjectedto the extraction is preferably made to be less than 1% by mass. Whenadditives are included, it is preferable to extract the additivestogether with the plasticizer in the step of extracting the plasticizer,and preferably the residual amount of the plasticizer in the membrane issubstantially 0%.

Preferably, the extraction solvent is a poor solvent for polyolefinresin and the inorganic particle, a good solvent for the plasticizer,and has a boiling point lower than the melting point of the polyolefinresin. Examples of such an extraction solvent include: hydrocarbons suchas n-hexane and cyclohexane; halogenated hydrocarbons such as methylenechloride and 1,1,1-trichloroethane; non-chlorine halogenated solventssuch as hydrofluoroether and hydrofluorocarbon; alcohols such as ethanoland isopropanol; ethers such as diethyl ether and tetrahydrofuran; andketones such as acetone and methyl ethyl ketone.

The addition of a heat treatment step such as thermal fixation orthermal relaxation is preferable because such treatment has a tendencyto further suppress the contraction of the obtained laminated separator.

It is also possible to further add a post-processing step. Examples ofthe post-processing step include hydrophilization treatment with anagent such as a surfactant and cross-linking treatment with ionizingradiation or the like.

For the laminated separator of Embodiment 1, the total membranethickness is preferably 2 μm or more, more preferably 5 μm or more,preferably 30 μm or less, more preferably 25 μm or less and furthermorepreferably 20 μm or less. The total membrane thickness set at 2 μm ormore is preferable from the viewpoint of improving the mechanicalstrength. On the other hand, the total membrane thickness set at 30 μmor less is preferable because such a membrane thickness reduces thevolume occupied by the separator to lead to a tendency to beadvantageous from the viewpoint of increasing battery capacities.

The lower limit of the thickness of each of the surface layers ispreferably 0.5 μm or more, more preferably 1 μm or more and furthermorepreferably 2 μm or more; the upper limit of the aforementioned thicknessis preferably 15 μm or less, more preferably 10 μm or less andfurthermore preferably 5 μm or less. The thickness of each of thesurface layers set at 0.5 μm or more is preferable from the viewpoint ofheat resistance. On the other hand, the thickness of each of the surfacelayers set at 15 μm or less is preferable from the viewpoint ofimproving the mechanical strength.

The porosity of the laminated separator is preferably 40% or more, morepreferably 50% or more, preferably 90% or less and more preferably 80%or less. The porosity set at 40% or more is preferable from theviewpoint of battery properties. On the other hand, the porosity set at90% or less is preferable from the viewpoint of ensuring the puncturestrength.

The air permeability of the laminated separator is preferably 10 sec/100cc or more, more preferably 50 sec/100 cc or more, preferably 1000sec/100 cc or less, more preferably 500 sec/100 cc or less andfurthermore preferably 300 sec/100 cc or less. The air permeability setat 10 sec/100 cc or more is preferable from the viewpoint of suppressingthe self-discharge of batteries. On the other hand, the air permeabilityset at 1000 sec/100 cc or less is preferable from the viewpoint ofobtaining satisfactory charge-discharge properties.

The puncture strength of the laminated separator is preferably 3.0 N/20μm or more, more preferably 4 N/20 μm or more, preferably 10 N/20 μm orless and more preferably 7 N/20 μm or less. The puncture strength set at3.0 N/20 μm or more is preferable from the viewpoint of suppressing themembrane breakage due to the causes such as the detached active materialat the time of battery winding. On the other hand, the puncture strengthset at 10 N/20 μm or less is preferable from the viewpoint of reducingthermal contraction.

The shutdown temperature as an index for the shutdown property of thelaminated separator is preferably 150° C. or lower, more preferably 145°C. or lower and further more preferably 140° C. or lower. The shutdowntemperature set at 150° C. or lower is preferable from the viewpoint ofensuring the safety of batteries.

The measurement value in a soldering test as the index for the heatresistance of the laminated separator is preferably 5.0 mm² or less andmore preferably 4.0 mm² or less at 300° C., and preferably 8.0 mm² orless and more preferably 7.0 mm² or less at 400° C. The measurementvalues in a soldering test set at 5.0 mm² or less at 300° C. and 8.0 mm²at 400° C. are preferable from the viewpoint of the heat resistance andthe uniform pore structure formation of a microporous membrane.

The capacity retention ratio as the index for the cycle property of thelaminated separator is preferably 80% or more and more preferably 85% ormore. The capacity retention ratio set at 80% or more is preferable fromthe viewpoint of the battery operation life.

The laminated separator of Embodiment 1 is useful as separators forbatteries, in particular, lithium ion batteries. Batteries obtained byusing the aforementioned laminated separator are excellent in cycleproperty and safety, and hence are useful for cellular phones,notebook-size personal computers, electric tools, hybrid vehicles andelectric vehicles.

Second Embodiment

The polyolefin microporous membrane of a second embodiment of thepresent invention (hereinafter, abbreviated as “Embodiment 2”) is formedof a polyolefin resin, as a main component, including 50 to 99% by massof polypropylene and 1 to 50% by mass of propylene-α-olefin copolymer,wherein the content of α-olefin in the propylene-α-olefin copolymer ismore than 1% by mass and 15% by mass or less.

In Embodiment 2, the separator having a satisfactory cycle property canbe realized by adopting the aforementioned composition, and the reasonsfor this are not clear; however, the reasons are inferred as follows.

Specifically, in Embodiment 2, polypropylene and propylene-α-olefincopolymer are mixed together, and it is inferred that the α-olefinfraction of the propylene-α-olefin copolymer has an action to decreasethe crystallinity of polypropylene. When a specific amount of theα-olefin fraction is included in the propylene-α-olefin copolymer, suchan α-olefin fraction appropriately acts on the polypropylene fraction,and thus probably regulates the lamellar layers formed in thepolypropylene fraction to have appropriate thinness. When a large numberof appropriately thin lamellar layers are formed, probably a largenumber of pores uniform in size are formed in the formation of amicroporous membrane. When such a microporous membrane having a largenumber of such pores uniform in size is disposed inside a battery, ionpermeation paths are present uniformly, without being sparse or dense,clogging hardly occurs in repeated charge-discharge cycles, and henceprobably the cycle property as a battery is improved.

Examples of the polypropylene include isotactic polypropylene andatactic polypropylene. These polypropylenes can be used each alone or asmixtures of two or more thereof.

The proportion of polypropylene in the polyolefin resin is 50% by massor more, preferably 60% by mass or more and more preferably 70% by massor more, and the upper limit of the aforementioned proportion is 99% bymass or less.

On the other hand, the propylene-α-olefin copolymer (simply described asthe “propylene copolymer,” as the case may be) is formed by usingpropylene as a monomer and α-olefin as another monomer different frompropylene.

Examples of such an α-olefin include ethylene, 1-butene,4-methyl-1-pentene, 1-hexene and 1-octene. In the propylene-α-olefincopolymer, the position of the α-olefin in the polymer chain is notparticularly limited, and either a random copolymer or a block copolymercan be used.

The content of the α-olefin in the propylene-α-olefin copolymer is morethan 1% by mass and 15% by mass or less, and is preferably 2% by mass ormore and 10% by mass or less. The aforementioned content set at morethan 1% by mass can contribute to the improvement of the batteryproperties. On the other hand, the aforementioned content set at 15% bymass or less is preferable from the viewpoint of improving the heatresistance of the obtained microporous membrane and improving the safetyof batteries.

The proportion of the propylene-α-olefin copolymer in the polyolefinresin is 1% by mass or more, more preferably 3% by mass or more andfurthermore preferably 5% by mass or more, and the upper limit of theaforementioned proportion is 50% by mass or less.

In the polyolefin resin, in addition to the aforementioned polypropyleneand propylene-α-olefin copolymer, other resin components may also bemixed.

Examples of such other resin components include the polymers(homopolymers and copolymer, and multistage polymers and the like)obtained by polymerizing the monomers such as ethylene, propylene,1-butene, 4-methyl-1-pentene, 1-hexene and 1-octene. The polymers can beused each alone or in combinations of two or more thereof. However, thecopolymer between polypropylene and the propylene-α-olefin copolymer areexcluded.

Examples of such other resin components also include polyethylene{low-density polyethylene (910 kg/m³ or more and less than 930 kg/m³),linear low-density polyethylene, medium-density polyethylene (930 kg/m³or more and less than 942 kg/m³), high-density polyethylene (942 kg/m³or more), ultra high molecular weight polyethylene} and polybutene.

The polyolefin resin preferably includes a high-density polyethylenefrom the viewpoint of improving the puncture strength of the obtainedpolyolefin microporous membrane.

The proportion of the high-density polyethylene in the polyolefin resinmay be 0% by mass, and is preferably 5% by mass or more, more preferably10% by mass or more and preferably 45% by mass or less. The proportionof the high-density polyethylene set at 45% by mass or less ispreferable from the viewpoint of improving the heat resistance andimproving the safety of batteries.

The viscosity average molecular weights (when two or more polyolefincomponents are used, the values measured for the respective polyolefincomponents are meant) of the aforementioned various polyolefincomponents are preferably 100,000 or more and more preferably 120,000 ormore, and the upper limits of the aforementioned molecular weights arepreferably 10,000,000 or less and more preferably 3,000,000 or less. Theaforementioned viscosity average molecular weights set at 100,000 ormore are preferable from the viewpoint of maintaining the high melttension at the time of melt molding and thus ensuring the satisfactorymoldability, or from the viewpoint of imparting sufficient entanglementand thus increasing the strength of the microporous membrane. On theother hand, the viscosity average molecular weights set at 10,000,000 orless are preferable from the viewpoint of realizing a uniformmelt-kneading and thus improving the moldability, in particular, thethickness stability of the sheet. The viscosity average molecularweights set at 3,000,000 or less are preferable from the viewpoint ofimproving the moldability.

Examples of the polyolefin resin include the same resins as listed inabove-described Embodiment 1. The mixing amounts of such resins are alsothe same as in Embodiment 1.

The polyolefin microporous membrane of Embodiment 2 includes as a maincomponent the polyolefin resin including polypropylene andpropylene-α-olefin copolymer. The term “main component” as referred toherein means that the proportion of the polyolefin resin in thepolyolefin microporous membrane is preferably 20% by mass or more, morepreferably 30% by mass or more, furthermore preferably 50% by mass ormore, yet furthermore preferably 70% by mass or more and particularlypreferably 90% by mass or more, and may also be 100% by mass.

The mixing ratio (polypropylene/propylene-α-olefin copolymer) (massratio) between the polypropylene and the propylene-α-olefin copolymer ispreferably 1.5 or more and 60 or less and more preferably 2 or more and55 or less. The aforementioned mixing ratio set at 60 or less ispreferable from the viewpoint of improving the battery properties, andthe aforementioned mixing ratio set at 1.5 or more is preferable fromthe viewpoint of improving the heat resistance and thus improving thesafety of batteries.

The polyolefin microporous membrane of Embodiment 2 may further includean inorganic filler.

Examples of such an inorganic filler include: oxide-based ceramics suchas alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria,yttria, zinc oxide and iron oxide; nitride-based ceramics such assilicon nitride, titanium nitride and boron nitride; ceramics such assilicon carbide, calcium carbonate, aluminum sulfate, aluminumhydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite,pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite,asbestos, zeolite, calcium silicate, magnesium silicate, diatom earthand silica sand; and glass fiber. These can be used each alone or incombinations of two or more thereof. From the viewpoint ofelectrochemical stability, preferable among these are silica, aluminaand titania, and particularly preferable is silica.

The average particle size of the inorganic filler is preferably 1 nm ormore, more preferably 6 nm or more and furthermore preferably 10 nm ormore, and the upper limit of the aforementioned average particle size ispreferably 100 nm or less, preferably 80 nm or less and furthermorepreferably 60 nm or less. The average particle size set at 100 nm orless leads to a tendency to make it difficult to cause the exfoliationbetween polyolefin and the inorganic filler even when an operation suchas stretching is performed, and is preferable from the viewpoint ofsuppressing the occurrence of macro voids. The difficulty in occurrenceof the exfoliation between polyolefin and the inorganic filler ispreferable from the viewpoint of highly hardening the fibrils themselvesconstituting the microporous membrane, and is also preferable because atendency to excel in the anti-compression performance in the localregions of the polyolefin microporous membrane or to excel in the heatresistance is observed. The cohesion between polyolefin and theinorganic filler is preferable from the viewpoint of realizing aseparator improving the affinity of the separator for an electricitystorage device to the nonaqueous electrolyte, and being excellent in theperformances such as the output power retention performance and thecycle retention performance. On the other hand, the average particlesize set at 1 nm or more is preferable from the viewpoint of ensuringthe dispersibility of the inorganic filler and improving theanti-compression property in the local regions.

When the polyolefin resin includes polyethylene, the mixing of theinorganic filler having an average particle size of 1 nm or more and 100nm or less in the composition including polyethylene and polypropyleneis preferable from the viewpoint of improving the compatibility betweenpolyethylene and polypropylene to suppress the phase separation betweenpolyethylene and polypropylene and thus ensuring satisfactorystretchability.

The plasticizer oil absorption amount of the inorganic filler ispreferably 150 ml/100 g or more, and the upper limit of theaforementioned oil absorption amount is preferably 1000 ml/100 g or lessand more preferably 500 ml/100 g or less. The aforementioned oilabsorption amount set at 150 ml/100 g or more is preferable from theviewpoint of suppressing the occurrence of aggregates in the kneadedmixture including polyolefin resin, the inorganic filler and theplasticizer and thus ensuring satisfactory moldability. Theaforementioned oil absorption amount set at 150 ml/100 g or more is alsopreferable from the viewpoint of being excellent in the impregnationproperty and the liquid retention property of nonaqueous electrolytes,and ensuring the productivity of electricity storage devices andensuring the performances in long-term use of electricity storagedevices when the polyolefin microporous membrane is used as theseparator for electricity storage devices. On the other hand, theaforementioned oil absorption amount set at 1000 ml/100 g or less ispreferable from the viewpoint of the handleability of the inorganicfiller at the time of producing the polyolefin microporous membrane.

The proportion of the inorganic filler in the total amount of theinorganic filler and the polyolefin resin is preferably 1% by mass ormore, more preferably 5% by mass or more and furthermore preferably 20%by mass or more, and the upper limit of the aforementioned proportion ispreferably 80% by mass or less, more preferably 60% by mass or less,furthermore preferably 50% by mass or less and particularly preferably40% by mass or less.

The aforementioned proportion set at 1% by mass or more is preferablefrom the viewpoint of forming the polyolefin microporous membrane so asto have a high porosity, the viewpoint of improving the thermalcontraction ratio of the polyolefin microporous membrane at 140° C. inthe transverse direction (widthwise direction, TD direction) of thepolyolefin microporous membrane, and moreover, the viewpoint ofregulating the membrane thickness retention ratio to be high and themembrane thickness reduction ratio to be small in penetration creep. Theaforementioned proportion set at 20% by mass or more is preferable fromthe viewpoint of improving the heat resistance.

On the other hand, the aforementioned proportion set at 80% by mass orless is preferable from the viewpoint of improving the membraneformability at a high stretching magnification and improving thepuncture strength of the polyolefin microporous membrane.

The volume proportion of the inorganic filler in the polyolefin resin ispreferably 10% by volume or more, more preferably 15% by volume andfurthermore preferably 20% by volume or more, and the upper limit of theaforementioned proportion is preferably 80% by volume.

The specific gravity of the inorganic filler is preferably 1.0 g/cm³ ormore, more preferably 1.2 g/cm³ or more and furthermore preferably 1.5g/cm³ or more, and the upper limit of the aforementioned specificgravity is preferably 10.0 g/cm³ or less.

The polyolefin microporous membrane of Embodiment 2 may further includevarious additives.

Examples of such additives include various additives such as: a phenolicantioxidant, a phosphorus-based antioxidant and a sulfur-basedantioxidant; metal soaps such as calcium stearate and zinc stearate; anultraviolet absorber; a light stabilizer, an antistatic agent, anantifogging agent and a pigment. The addition amounts of these additivesare preferably 0.01% by mass or more and 1% by mass or less based on thecomposition (hereinafter, also referred to as the “polyolefincomposition”) including polyolefin resin.

In the laminated polyolefin microporous membrane of Embodiment 2, on atleast one side of the above-described polyolefin microporous membrane,another polyolefin microporous membrane which is different from theabove-described polyolefin microporous membrane is laminated. Theformation of such a laminated polyolefin microporous membrane ispreferable from the viewpoint of imparting other performances such aslow fuse function. From the viewpoint of the productivity, morepreferable is an aspect of the two-type three-layer structure in whichthe two layers as the surface layers are the same in composition and theintermediate layer is different in composition from these two layers.

As the other polyolefin microporous membrane, heretofore knownmicroporous membranes can be used.

The method for producing the polyolefin microporous membrane or thelaminated polyolefin microporous membrane is not particularly limited,and for example, a method including the following steps (1) to (5) canbe used:

(1) a kneading step of forming, according to an intended layerstructure, a kneaded mixture by kneading a polyolefin composition and aplasticizer which are the raw materials of each layer;

(2) a sheet molding step, following the kneading step, of processing thekneaded mixture into a sheet-like molded body (a monolayer molded bodyor a laminated molded body) according to an intended layer structure byextruding the kneaded mixture into a sheet and, where necessary, bylaminating the resulting sheets into a laminated body, and cooling andsolidifying the resulting monolayer body or the resulting laminatedbody;

(3) a stretching step, following the molding step, of forming astretched product by biaxially stretching the sheet-like molded bodywith an area magnification of 20× or more and 200× or less;

(4) a porous body forming step, following the stretching step, offorming a porous body by extracting the plasticizer from the stretchedproduct; and

(5) a heat treatment step, following the porous body forming step, ofheat treating the porous body at a temperature equal to or lower thanthe melting point of the polyolefin resin and widthwise stretching theporous body.

The plasticizer used in the step (1) is preferably a nonvolatile solventcapable of forming a uniform solution, when mixed with the polyolefinresin, at a temperature equal to or higher than the melting point of thepolyolefin resin. Additionally, the plasticizer is preferably liquid atnormal temperature.

Examples of the plasticizer include: hydrocarbons such as liquidparaffin and paraffin wax; esters such as diethylhexyl phthalate anddibutyl phthalate; and higher alcohols such as oleyl alcohol and stearylalcohol.

In particular, when the polyolefin resin includes polyethylene, the useof liquid paraffin as a plasticizer is preferable from the viewpoint ofsuppressing the exfoliation at the interface between the polyolefinresin and the plasticizer and thus performing uniform stretching, or theviewpoint of realizing a high puncture strength. The use of diethylhexylphthalate is preferable from the viewpoint of increasing the load at thetime of melt-extruding the kneaded mixture and improving thedispersibility of the inorganic filler (realizing a good qualitymembrane).

The proportion of the plasticizer in the kneaded mixture is preferably25% by mass or more and more preferably 30% by mass or more, and theupper limit of the aforementioned proportion is preferably 80% by massor less and preferably 75% by mass or less. The aforementionedproportion set at 80% by mass or less is preferable from the viewpointof maintaining the high melt tension at the time of melt-molding andensuring the moldability. On the other hand, the aforementionedproportion set at 25% by mass or more is preferable from the viewpointof ensuring the moldability and from the viewpoint of efficientlystretching the lamellar crystals in the crystalline regions ofpolyolefin. The efficient stretching of the lamellar crystals means theefficient stretching of the polyolefin chain without causing thebreakage of the polyolefin chain, and such an efficient stretching cancontribute to the formation of uniform and fine pore structure and tothe improvement of the strength and the crystallinity of the polyolefinmicroporous membrane.

Examples of the method for kneading polyolefin resin, an inorganicfiller and a plasticizer include the following methods (a) and (b).

(a) A method in which polyolefin resin and the inorganic filler areplaced in a resin kneading apparatus such as an extruder or a kneader,and a plasticizer is further introduced and kneaded while the resin isbeing heated, melted and kneaded.

(b) A method in which polyolefin resin, an inorganic filler and aplasticizer are preliminarily kneaded in a predetermined ratio by usinga Henschel mixer or the like, and after such a preliminary kneadingstep, the resulting kneaded mixture is placed in an extruder, and aplasticizer is introduced into the mixture and the mixture is furtherkneaded while the mixture is being heated and melted.

In the preliminary kneading in the method (b), from the viewpoint ofimproving the dispersibility of the inorganic filler and performing ahigh-magnification stretching without causing the breakage of themembrane, it is preferable to perform the preliminary kneading by mixingthe plasticizer in an amount specified by the range of the followingformula (1) based on the polyolefin resin and the inorganic filler:

0.6≦weight of plasticizer/(plasticizer oil absorption amount×weight ofinorganic filler×density of plasticizer)×100≦1.2  (1)

The step (2) is a step in which the kneaded mixture is extruded, forexample, into a sheet shape through a T-die and the resultingsheet-shaped product is brought into contact with a heat conductor to becooled and solidified and thus a gel sheet is obtained. When a laminatedgel sheet is formed, such a laminated gel sheet can be prepared eitherby a method in which the gel sheets forming the respective layers areintegrated and coextruded from the respective extruders through a die,or a method in which the gel sheets forming the respective layers aresuperposed and thermally fusion bonded. Of these two methods, the methodperforming coextrusion is more preferable because a high interlayeradhesion strength is easily obtained by this method, and this methodfacilitates the formation of continuous pores between the layers,accordingly facilitates the maintenance of a high permeability, and isexcellent in productivity. As the aforementioned heat conductor, ametal, water, air or the plasticizer itself can be used. The cooling andsolidification performed by putting a sheet between rolls is preferablefrom the viewpoint of increasing the membrane strength of the sheet-likemolded body and improving the surface smoothness of the sheet-likemolded body.

Examples of the stretching method in the step (3) include simultaneousbiaxial stretching, successive biaxial stretching, multiple stagestretching and multiple time stretching. Adoption of simultaneousbiaxial stretching among these is preferable from the viewpoint ofincreasing the puncture strength and uniformizing the membrane thicknessof the polyolefin microporous membrane.

The area magnification in the step (3) is preferably 10 times or moreand preferably 15 times or more, and the upper limit of theaforementioned area magnification is preferably 200 times or less andmore preferably 100 times or less. The aforementioned area magnificationset at 10 times or more is preferable from the viewpoint of ensuringsufficient strength as a separator.

The stretching temperature in the step (3) is, by taking the meltingpoint of the polyolefin resin as a reference temperature, preferably themelting point−50° C. or higher, more preferably the melting point−30° C.or higher and furthermore preferably the melting point−20° C. or higher,and the upper limit of the aforementioned stretching temperature ispreferably the melting point−2° C. or lower and more preferably themelting point−3° C. or lower. The aforementioned stretching temperatureset at the melting point−50° C. or higher is preferable from theviewpoint of achieving a satisfactory adhesion in the interface betweenthe polyolefin resin and the inorganic filler or between the polyolefinresin and the plasticizer and thus improving the anti-compressionperformance, in local and microscopic regions, of the polyolefinmicroporous membrane. For example, when a high-density polyethylene isused as polyolefin resin, the stretching temperature is preferably 115°C. or higher and 132° C. or lower. When a mixture of two or morepolyolefins is used, the melting point of the polyolefin having thehighest heat of fusion can be taken as the reference.

The step (4) is preferably performed after the step (3) from theviewpoint of improving the puncture strength of the polyolefinmicroporous membrane. Examples of the extraction method include a methodin which the stretched product is immersed in the solvent for theplasticizer. The residual amount of the plasticizer in the microporousmembrane after the extraction is preferably regulated to be 1% by massor less.

The step (5) is preferably a step in which thermal fixation and/or thethermal relaxation is performed.

The stretching magnification in the step (5) is, in terms of the areamagnification, preferably less than 4 times and more preferably lessthan 3 times. The aforementioned area magnification set at less than 4times is preferable from the viewpoint of suppressing the occurrence ofmacro voids and the decrease of the puncture strength.

The heat treatment temperature is, by taking the melting point of thepolyolefin resin as the reference, preferably 100° C. or higher, and theupper limit of the aforementioned heat treatment temperature ispreferably equal to or lower than the melting point of the polyolefin.The heat treatment temperature set at 100° C. or higher is preferablefrom the viewpoint of suppressing the occurrence of the membranebreakage. On the other hand, the heat treatment temperature set at atemperature equal to or lower than the melting point of the polyolefinis preferable from the viewpoint of suppressing the contraction of thepolyolefin resin and reducing the thermal contraction ratio of thepolyolefin microporous membrane.

After the step (5), the obtained polyolefin microporous membrane may besubjected to a post treatment. Examples of such a post treatment includea hydrophilization treatment with an agent such as a surfactant andcross-linking treatment with ionizing radiation or the like.

The puncture strength of the polyolefin microporous membrane or thelaminated polyolefin microporous membrane (simply abbreviated as the“microporous membrane,” as the case may be) of Embodiment 2 ispreferably 200 g/20 μm or more, more preferably 240 g/20 μm or more andfurthermore preferably 300 g/20 μm or more, and the upper limit of theaforementioned puncture strength is preferably 2000 g/20 μm or less andmore preferably 1000 g/20 μm or less. The aforementioned puncturestrength set at 200 g/20 μm or more is preferable from the viewpoint ofsuppressing the membrane breakage due to the causes such as the detachedactive material at the time of battery winding, and is also preferablefrom the viewpoint of suppressing an adverse possibility thatshort-circuit occurs due to the expansion and contraction of theelectrodes associated with charging and discharging. On the other hand,the aforementioned puncture strength set at 2000 g/20 μm or less ispreferable from the viewpoint of being capable of reducing the widthcontraction due to the orientation relaxation at the time of heating.

The puncture strength can be regulated by the operations such ascontrolling the stretching magnification and the stretching temperature.

The porosity of the microporous membrane is preferably 20% or more andmore preferably 35% or more, and the upper limit of the aforementionedporosity is preferably 90% or less and more preferably 80% or less. Theaforementioned porosity set at 20% or more is preferable form theviewpoint of ensuring the permeability of the separator. On the otherhand, the aforementioned porosity set at 90% or less is preferable fromthe viewpoint of ensuring the puncture strength.

The porosity can be regulated by variation of the stretchingmagnification.

The average pore size of the microporous membrane is preferably 0.1 μmor less and more preferably 0.08 μm or less, and the lower limit of theaforementioned pore size is preferably 0.01 μm or more. Theaforementioned average pore size set at 0.1 μm or less is preferablefrom the viewpoint of suppressing the self-discharge of electricitystorage devices and thus suppressing the capacity reduction.

The aforementioned average pore size can be regulated by variation ofthe stretching magnification.

The membrane thickness of the microporous membrane is preferably 2 μm ormore and more preferably 5 μm or more, and the upper limit of theaforementioned membrane thickness is preferably 100 μm or less, morepreferably 60 μm or less and furthermore preferably 50 μm or less. Theaforementioned membrane thickness set at 2 μm or more is preferable fromthe viewpoint of improving the mechanical strength. On the other hand,the aforementioned membrane thickness set at 100 μm or less ispreferable because such a membrane thickness reduces the volume occupiedby the separator to lead to a tendency to be advantageous from theviewpoint of increasing battery capacities.

The air permeability of the microporous membrane is preferably 10sec/100 cc or more and more preferably 50 sec/100 cc or more, and theupper limit of the aforementioned air permeability is preferably 1000sec/100 cc or less and more preferably 500 sec/100 cc or less. Theaforementioned air permeability set at 10 sec/100 cc or more ispreferable from the viewpoint of suppressing the self-discharge ofelectricity storage devices. On the other hand, the aforementioned airpermeability set at 1000 sec/100 cc or less is preferable from theviewpoint of obtaining satisfactory charge-discharge properties.

The aforementioned air permeability can be regulated by operations suchas varying the stretching temperature and the stretching magnification.

The polyolefin microporous membrane of Embodiment 2 is useful asseparators for electricity storage devices, in particular. Usually, anelectricity storage device uses the aforementioned microporous membranefor the separator and includes a positive electrode, a negativeelectrode and an electrolyte.

The electricity storage device can be produced, for example, as follows:the microporous membrane is prepared, for example, as an oblongrectangular separator of 10 to 500 mm (preferably 80 to 500 mm) in widthand 200 to 4000 m (preferably 1000 to 4000 m) in length; by using theprepared separator, a four-layer assembly is prepared by superposing apositive electrode, the separator, a negative electrode and theseparator in this order, or a negative electrode, the separator, apositive electrode and the separator in this order; the four-layerassembly is wound into a cylindrical roll or an oblate cylindrical rollto yield a wound body; the wound body is housed in a battery can; andfurther, an electrolyte is injected into the battery to produce theaforementioned electricity storage device. Alternatively, theelectricity storage device can also be produced as follows: a four-layerlaminate is prepared by laminating, into a flat form, a positiveelectrode, the separator, a negative electrode and the separator in thisorder, or a negative electrode, the separator, a positive electrode andthe separator in this order; the four-layer laminate is laminated with abag-shaped film and an electrolyte is injected into the bag to producethe electricity storage device.

Third Embodiment

The polyolefin microporous membrane of a third embodiment of the presentinvention (hereinafter, abbreviated as “Embodiment 3”) is formed of apropylene-based resin composition including a polypropylene-based resinhaving the (polypropylene)/(propylene copolymer) (mass ratio) of 80/20to 0/100 as a main component, wherein the melting point of the propylenecopolymer is 120° C. or higher and 145° C. or lower.

In Embodiment 3, the separator having a satisfactory cycle property canbe realized by adopting the aforementioned composition, and the reasonsfor this are not clear; however, the reasons are inferred as follows.

Specifically, in Embodiment 3, polypropylene and a propylene copolymer(propylene-ethylene copolymer or propylene-α-olefin copolymer) having aspecific melting point are mixed together, and it is inferred that theethylene fraction of the propylene-ethylene copolymer and the α-olefinfraction of the propylene-α-olefin copolymer each have an action todecrease the crystallinity of the polypropylene. Accordingly, theethylene fraction of the propylene-ethylene copolymer and the α-olefinfraction of the propylene-α-olefin copolymer appropriately acts on thepolypropylene fraction, and thus probably regulates the lamellar layersformed in the polypropylene fraction to have appropriate thinness. Whena large number of appropriately thin lamellar layers are formed,probably a large number of pores uniform in size are formed in theformation of a microporous membrane.

Probably, by setting the melting point of the propylene-ethylenecopolymer or the propylene-α-olefin random copolymer so as to fallwithin a specific range of 120° C. or higher and 145° C. or lower, thestretchability of the composition as a whole is improved and the poresize is more increased.

When such a microporous membrane having a large number of such poreslarge and uniform in size is disposed inside a battery, ion permeationpaths are present uniformly, without being sparse or dense, clogginghardly occurs in repeated charge-discharge cycles, and hence probablythe cycle property as a battery is improved.

The polyolefin microporous membrane of Embodiment 3 is formed of apolypropylene-based resin composition including, as a main component, apolypropylene-based resin including polypropylene and propylenecopolymer.

Examples of the polypropylene include isotactic polypropylene andatactic polypropylene. These polypropylenes can be used each alone or asmixtures of two or more thereof.

The heat of fusion of the polypropylene is preferably 80 J/g or more,more preferably 85 J/g or more and furthermore preferably 90 J/g ormore. The heat of fusion of the polypropylene set at 80 J/g or more ispreferable from the viewpoint of improving the porosity.

On the other hand, the propylene copolymer (propylene-ethylene copolymeror propylene-α-olefin copolymer) is formed by using propylene as amonomer and ethylene or α-olefin as another monomer different frompropylene.

Examples of such an α-olefin include 1-butene, 4-methyl-1-pentene,1-hexene and 1-octene. In the propylene-ethylene copolymer or thepropylene-α-olefin copolymer, the position of the ethylene or theα-olefin in the polymer chain may be either of a block copolymer or of arandom copolymer; however, the random copolymer is preferable from theviewpoint of improving the stretchability and increasing the pore size.

The heat of fusion of the propylene-ethylene copolymer or thepropylene-α-olefin copolymer is preferably 60 J/g or more, morepreferably 65 J/g or more and furthermore preferably 70 J/g or more,from the viewpoint of improving the porosity of the polyolefinmicroporous membrane.

The heat of fusion of the propylene-ethylene copolymer or thepropylene-α-olefin copolymer set at 60 J/g or more is preferable fromthe viewpoint of improving the porosity.

The content of ethylene or α-olefin in the propylene copolymer is morethan 1% by mass and 20% by mass or less, and is preferably 2% by mass ormore and 18% by mass or less. The aforementioned content set at morethan 1% by mass is capable of contributing to the improvement of thebattery properties. On the other hand, the aforementioned content set at20% by mass or less is preferable from the viewpoint of improving theheat resistance of the obtained microporous membrane and improving thesafety of batteries.

The proportion of the propylene copolymer in the polypropylene-basedresin is 20% by mass or more, more preferably 25% by mass or more andfurthermore preferably 30% by mass or more, and the upper limit of theaforementioned proportion is 100% by mass or less.

The aforementioned proportion set at 20% by mass or more is preferablefrom the viewpoint of uniformizing and increasing in size the pores ofthe polyolefin microporous membrane and improving the membraneformability at a high stretching magnification.

The melting point of the propylene copolymer is 120° C. or higher,preferably 122° C. or higher and furthermore preferably 125° C. orhigher, and the upper limit of the aforementioned melting point is 145°C. or lower, preferably 143° C. or lower and furthermore preferably 140°C. or lower.

The melting point of the aforementioned propylene copolymer set at 120°C. or higher is preferable from the viewpoint of improving thestretchability.

In the polypropylene-based resin composition, in addition to theaforementioned polypropylene and propylene copolymer, other resincomponents may also be mixed.

Examples of such other resin components include the same resincomponents as listed in Embodiment 2.

The polypropylene-based resin composition preferably includes ahigh-density polyethylene from the viewpoint of improving the puncturestrength of the obtained polyolefin microporous membrane.

The proportion of the high-density polyethylene in thepolypropylene-based resin composition is preferably 5% by mass or moreand more preferably 10% by mass or more, and the upper limit of theaforementioned proportion is preferably 50% by mass or less. Theproportion of the high-density polyethylene set at 50% by mass or lessis preferable from the viewpoint of improving the heat resistance andimproving the safety of batteries.

The proportion of the polypropylene in the polypropylene-based resincomposition is preferably 0% by mass or more and more preferably 20% bymass or more, and the upper limit of the aforementioned proportion is80% by mass or less. The proportion of polypropylene set at 80% by massor less is preferable from the viewpoint of improving thestretchability.

The viscosity average molecular weights (when two or more components areused, the values measured for the respective components are meant) ofthe aforementioned various components are preferably 100,000 or more andmore preferably 120,000 or more, and the upper limits of theaforementioned molecular weights are preferably 10,000,000 or less andmore preferably 3,000,000 or less. The aforementioned viscosity averagemolecular weights set at 100,000 or more are preferable from theviewpoint of maintaining the high melt tension at the time of meltmolding and thus ensuring the satisfactory moldability, or from theviewpoint of imparting sufficient entanglement and thus increasing thestrength of the microporous membrane. On the other hand, the viscosityaverage molecular weights set at 10,000,000 or less are preferable fromthe viewpoint of realizing a uniform melt-kneading and thus improvingthe moldability, in particular, the thickness stability of the sheet.The viscosity average molecular weights set at 3,000,000 or less arepreferable from the viewpoint of improving the moldability.

Examples of the polyolefin resin include the same resins as listed inabove-described Embodiment 1. The mixing amounts of such resins are alsothe same as in Embodiment 1.

The polyolefin microporous membrane of Embodiment 3 preferably includesas a main component the polypropylene-based resin. The term “maincomponent” as referred to herein means that the proportion of thepolypropylene-based resin in the polypropylene-based resin compositionis preferably 20% by mass or more, more preferably 30% by mass or more,furthermore preferably 40% by mass or more and particularly preferably45% by mass or more. The upper limit of the aforementioned proportion ispreferably 100% by mass or less.

The polyolefin microporous membrane of Embodiment 3 may further includean inorganic filler.

Examples of such an inorganic filler include the same inorganic fillersas listed in Embodiment 2.

The primary particle size of the inorganic filler is preferably 1 nm ormore, more preferably 6 nm or more and furthermore preferably 10 nm ormore, and the upper limit of the aforementioned primary particle size ispreferably 100 nm or less, preferably 80 nm or less and furthermorepreferably 60 nm or less. The primary particle size set at 100 nm orless leads to a tendency to make it difficult to cause the exfoliationbetween polyolefin and the inorganic particles even when an operationsuch as stretching is performed, and is preferable from the viewpoint ofsuppressing the occurrence of macro voids. The difficulty in occurrenceof the exfoliation between polyolefin and the inorganic filler ispreferable from the viewpoint of highly hardening the fibrils themselvesconstituting the microporous membrane, and is also preferable because atendency to excel in the anti-compression performance in the localregions of the polyolefin microporous membrane or to excel in the heatresistance is observed. The cohesion between polyolefin and theinorganic filler is preferable from the viewpoint of realizing aseparator improving the affinity of the separator for an electricitystorage device to the nonaqueous electrolyte, and being excellent in theperformances such as the output power retention performance and thecycle retention performance.

The plasticizer oil absorption amount of the inorganic filler ispreferably 150 ml/100 g or more, and the upper limit of theaforementioned oil absorption amount is preferably 1000 ml/100 g or lessand more preferably 500 ml/100 g or less. The aforementioned oilabsorption amount set at 150 ml/100 g or more is preferable from theviewpoint of suppressing the occurrence of aggregates in the kneadedmixture including polyolefin resin, the inorganic filler and theplasticizer and thus ensuring satisfactory moldability. Theaforementioned oil absorption amount set at 150 ml/100 g or more is alsopreferable from the viewpoint of being excellent in the impregnationproperty and the liquid retention property of nonaqueous electrolytes,and ensuring the productivity of electricity storage devices andensuring the performances in long-term use of electricity storagedevices when the polyolefin microporous membrane is used as theseparator for electricity storage devices. On the other hand, theaforementioned oil absorption amount set at 1000 ml/100 g or less ispreferable from the viewpoint of the handleability of the inorganicfiller at the time of producing the polyolefin microporous membrane.

The proportion of the inorganic filler in the polyolefin-based resincomposition is preferably 5% by mass or more, more preferably 10% bymass or more and furthermore preferably 20% by mass or more, and theupper limit of the aforementioned proportion is preferably 60% by massor less, more preferably 50% by mass or less and furthermore preferably40% by mass or less.

The aforementioned proportion set at 5% by mass or more is preferablefrom the viewpoint of forming the polyolefin microporous membrane so asto have a high porosity, the viewpoint of improving the thermalcontraction ratio of the polyolefin microporous membrane at 140° C. inthe transverse direction (widthwise direction, TD direction) of thepolyolefin microporous membrane, and moreover, the viewpoint ofregulating the membrane thickness retention ratio to be high and themembrane thickness reduction ratio to be small in penetration creep. Theaforementioned proportion set at 20% by mass or more is preferable fromthe viewpoint of improving the heat resistance.

On the other hand, the aforementioned proportion set at 60% by mass orless is preferable from the viewpoint of improving the membraneformability at a high stretching magnification and improving thepuncture strength of the polyolefin microporous membrane.

The volume proportion of the inorganic filler in the propylene-basedresin composition is preferably 10% by volume or more, more preferably15% by volume or more and furthermore preferably 20% by volume or more,and the upper limit of the aforementioned proportion is preferably 80%by volume or less.

The specific gravity of the inorganic filler is preferably 1.0 g/cm³ ormore, more preferably 1.2 g/cm³ or more and furthermore preferably 1.5g/cm³ or more, and the upper limit of the aforementioned specificgravity is preferably 10.0 g/cm³ or less.

The propylene-based resin composition may further include variousadditives.

Examples of such additives include the same additives as listed inEmbodiment 2.

In the laminated polyolefin microporous membrane of Embodiment 3, on atleast one side of the above-described polyolefin microporous membrane,another polyolefin microporous membrane different from theabove-described polyolefin microporous membrane is laminated. Theformation of such a laminated polyolefin microporous membrane ispreferable from the viewpoint of imparting other performances such aslow fuse function. From the viewpoint of the productivity, morepreferable is an aspect of the two-type three-layer structure in whichthe two layers as the surface layers are the same in composition and theintermediate layer is different in composition from these two layers.

As the other polyolefin microporous membrane, heretofore knownmicroporous membranes can be used.

As the method for producing the polyolefin microporous membrane or thelaminated polyolefin microporous membrane, the same production method asin Embodiment 2 can be used.

For the polyolefin microporous membrane or the laminated polyolefinmicroporous membrane (simply abbreviated as the “microporous membrane,”as the case may be) of Embodiment 3, the appropriate numerical ranges ofthe puncture strength, the porosity, the average pore size, the membranethickness and the air permeability are the same as in Embodiment 2.

The capacity retention ratio as the index for the cycle property of thelaminated separator is preferably 70% or more and more preferably 75% ormore. The capacity retention ratio set at 70% or more is preferable fromthe viewpoint of the battery operation life.

The high-temperature storage property of the laminated separator ispreferably 60% or more and more preferably 65%. The high-temperaturestorage property set at 65% or more is preferable from the viewpoint ofthe battery operation life.

The short-circuit temperature as an index for the heat resistance of thelaminated separator is preferably 160° C. or higher and more preferably165° C. or higher. The short-circuit temperature set at 160° C. orhigher is preferable from the viewpoint of the safety of batteries.

The microporous membrane is useful as separators for electricity storagedevices, in particular. Usually, an electricity storage device uses theaforementioned microporous membrane for the separator and includes apositive electrode, a negative electrode and an electrolyte.

The electricity storage device can be produced in the same manner as inabove-described Embodiment 2.

Fourth Embodiment

The polyolefin microporous membrane of a fourth embodiment of thepresent invention (hereinafter, abbreviated as “Embodiment 4”) is formedof a polypropylene-based resin composition including 20 to 95% by massof a polypropylene-based resin having (polypropylene)/(propylenecopolymer) (mass ratio) of 90/10 to 0/100 and 5 to 80% by mass of aninorganic filler, wherein the melting point of the propylene copolymeris 110° C. to 150° C., and the (propylene copolymer/inorganic filler)(mass ratio) is 0.1/1 to 1.5/1.

The “propylene copolymer” means “propylene-ethylene copolymer orpropylene-α-olefin copolymer.”

In Embodiment 4, the separator having a satisfactory cycle property canbe realized by adopting the aforementioned composition, and the reasonsfor this are not clear; however, the reasons are inferred as follows.

Specifically, in Embodiment 4, in the presence of polypropylene, apropylene copolymer having a specific melting point and an inorganicfiller are mixed. When such a specific propylene copolymer is mixed in apredetermined amount based on the inorganic filler, the propylenecopolymer can satisfactorily interact with polypropylene and theinorganic filler in the melting temperature region of polypropylene, andconsequently, the fluidity of the resin as a whole in the molten statecan be degraded. It is inferred that even when low molecular weightcomponents and/or polymer components low in crystallinity are includedin the raw material, the degradation of the fluidity of the resin as awhole suppresses the migration, to the surface of the microporousmembrane, of such components during the membrane formation, and thusleads to the difficulty in forming a skin layer. The formation of theskin layer leads to the impairment of the pore structure (blocking ofthe pores) present in the surface of the microporous membrane. In otherwords, in Embodiment 4, there can be realized a microporous membrane inwhich the skin layer formation is suppressed, the pores in the surfacelayer are not blocked and a large number of pores uniform in size areprovided. When such a microporous membrane is disposed inside a battery,ion permeation paths are present uniformly, without being sparse ordense, clogging hardly occurs in repeated charge-discharge cycles, andhence probably the cycle property as a battery is improved.

The polyolefin microporous membrane of Embodiment 4 is formed of apolypropylene-based resin composition including a polypropylene-basedresin including polypropylene and a propylene copolymer, and aninorganic filler.

Examples of the polypropylene include isotactic polypropylene andatactic polypropylene. These polypropylenes can be used each alone or asmixtures of two or more thereof.

From the viewpoint of improving the high-temperature storage propertyand the membrane formability of the polyolefin microporous membrane, theMFR (meaning “melt flow rate,” this also being the case hereinafter) ofthe polypropylene is preferably 0.1 g/10 min or more and 10.0 g/10 minor less and more preferably 8.0 g/min or less and furthermore preferably5.0 g/min or less.

The heat of fusion of the polypropylene is preferably 80 J/g or more,more preferably 85 J/g or more and furthermore preferably 90 J/g ormore. The heat of fusion of the polypropylene set at 80 J/g or more ispreferable from the viewpoint of improving the porosity.

On the other hand, the propylene copolymer (propylene-ethylene copolymeror propylene-α-olefin copolymer) is formed by using propylene as amonomer and ethylene or α-olefin as another monomer different frompropylene.

Examples of such an α-olefin include 1-butene, 4-methyl-1-pentene,1-hexene and 1-octene. In the propylene-ethylene copolymer or thepropylene-α-olefin copolymer, the position of the ethylene or theα-olefin in the polymer chain may be either of a block copolymer or of arandom copolymer; however, the random copolymer is preferable.

The proportion of the propylene copolymer in the propylene-based resinis 10% by mass or more, more preferably 15% by mass or more andfurthermore preferably 20% by mass or more, and the upper limit of theaforementioned proportion is preferably 100% by mass or less, morepreferably 80% by mass or less and furthermore preferably 60% by mass orless.

The aforementioned proportion set at 10% by mass or more is preferablefrom the viewpoint of uniformizing and increasing in size the pores ofthe polyolefin microporous membrane and improving the membraneformability at a high stretching magnification.

The melting point of the propylene copolymer is 110° C. or higher,preferably 115° C. or higher and furthermore preferably 120° C. orhigher, and the upper limit of the aforementioned melting point is 150°C. or lower, preferably 147° C. or lower and furthermore preferably 145°C. or lower.

The melting point of the aforementioned propylene copolymer set at 110°C. or higher is preferable from the viewpoint of improving thestretchability.

From the viewpoint of improving the membrane formability of thepolyolefin microporous membrane, the MFR of the propylene copolymer ispreferably 0.1 g/10 min or more and 20.0 g/10 min or less, morepreferably 15.0 g/10 min or less and furthermore preferably 10.0 g/10min.

The heat of fusion of the propylene copolymer is 60 J/g or more, morepreferably 65 J/g or more and furthermore preferably 70 J/g or more.

The heat of fusion of the propylene copolymer set at 60 J/g or more ispreferable from the viewpoint of improving the porosity.

The content of the comonomer (the content of ethylene and α-olefin) inthe propylene copolymer is more than 1% by mass and is 20% by mass orless and is preferably 2% by mass or more and 18% by mass or less. Theaforementioned content set at more than 1% by mass can contribute to theimprovement of the battery properties. On the other hand, theaforementioned content set at 20% by mass or less is preferable from theviewpoint of improving the heat resistance of the obtained microporousmembrane and improving the safety of batteries.

In the polypropylene-based resin composition, in addition to theaforementioned polypropylene and propylene copolymer, other resincomponents may also be mixed.

Examples of such other resin components include the same resincomponents as listed in Embodiment 2.

The polypropylene-based resin composition preferably includes ahigh-density polyethylene from the viewpoint of improving the puncturestrength of the obtained polyolefin microporous membrane.

The proportion of the high-density polyethylene in thepolypropylene-based resin composition is preferably 5% by mass or moreand more preferably 10% by mass or more, and the upper limit of theaforementioned proportion is preferably 60% by mass or less. Theproportion of the high-density polyethylene set at 60% by mass or lessis preferable from the viewpoint of improving the heat resistance andimproving the safety of batteries.

The viscosity average molecular weights (when two or more components areused, the values measured for the respective components are meant) ofthe aforementioned various components are preferably 100,000 or more andmore preferably 120,000 or more, and the upper limits of theaforementioned molecular weights are preferably 10,000,000 or less andmore preferably 3,000,000 or less. The aforementioned viscosity averagemolecular weights set at 100,000 or more are preferable from theviewpoint of maintaining the high melt tension at the time of meltmolding and thus ensuring the satisfactory moldability, or from theviewpoint of imparting sufficient entanglement and thus increasing thestrength of the microporous membrane. On the other hand, the viscosityaverage molecular weights set at 10,000,000 or less are preferable fromthe viewpoint of realizing a uniform melt-kneading and thus improvingthe moldability, in particular, the thickness stability of the sheet.The viscosity average molecular weights set at 3,000,000 or less arepreferable from the viewpoint of improving the moldability.

Examples of the polyolefin resin include the same resins as listed inabove-described Embodiment 1. The mixing amounts of such resins are alsothe same as in Embodiment 1.

The polyolefin microporous membrane of Embodiment 4 preferably includesthe polypropylene-based resin as a main component. The term “maincomponent” as referred to herein means that the proportion of thepolypropylene-based resin in the polypropylene-based resin compositionis preferably 20% by mass or more, more preferably 30% by mass or more,furthermore preferably 40% by mass or more and particularly preferably45% by mass or more. The upper limit of the aforementioned proportion ispreferably 95% by mass or less.

The polyolefin microporous membrane of Embodiment 4 further includes aninorganic filler.

Examples of such an inorganic filler include the same inorganic fillersas listed in Embodiment 2.

The average particle size of the inorganic filler is preferably 1 nm ormore, more preferably 6 nm or more and furthermore preferably 10 nm ormore, and the upper limit of the aforementioned average particle size ispreferably 100 nm or less, preferably 80 nm or less and furthermorepreferably 60 nm or less. The average particle size set at 100 nm orless leads to a tendency to make it difficult to cause the exfoliationbetween polyolefin and the inorganic particles even when an operationsuch as stretching is performed, and is preferable from the viewpoint ofsuppressing the occurrence of macro voids. The difficulty in occurrenceof the exfoliation between polyolefin and the inorganic filler ispreferable from the viewpoint of highly hardening the fibrils themselvesconstituting the microporous membrane, and is also preferable because atendency to excel in the anti-compression performance in the localregions of the polyolefin microporous membrane or to excel in the heatresistance is observed. The cohesion between polyolefin and theinorganic filler is preferable from the viewpoint of realizing aseparator improving the affinity of the separator for an electricitystorage device to the nonaqueous electrolyte, and being excellent in theperformances such as the output power retention performance and thecycle retention performance.

On the other hand, the average particle size set at 1 nm or more ispreferable from the viewpoint of ensuring the dispersibility of theinorganic filler and improving the anti-compression property in thelocal regions.

The mixing of the inorganic particles having a particle size of 1 nm ormore and 100 nm or less in the composition including polyethylene andpolypropylene is preferable from the viewpoint of improving thecompatibility between polyethylene and polypropylene to suppress thephase separation between polyethylene and polypropylene and thusensuring satisfactory stretchability.

The plasticizer oil absorption amount of the inorganic filler ispreferably 150 ml/100 g or more, and the upper limit of theaforementioned oil absorption amount is preferably 1000 ml/100 g or lessand more preferably 500 ml/100 g or less. The aforementioned oilabsorption amount set at 150 ml/100 g or more is preferable from theviewpoint of suppressing the occurrence of aggregates in the kneadedmixture including the polyolefin resin, the inorganic filler and theplasticizer and thus ensuring satisfactory moldability. Theaforementioned oil absorption amount set at 150 ml/100 g or more is alsopreferable from the viewpoint of being excellent in the impregnationproperty and the liquid retention property of nonaqueous electrolytes,and ensuring the productivity of electricity storage devices andensuring the performances in long-term use of electricity storagedevices when the polyolefin microporous membrane is used as theseparator for electricity storage devices. On the other hand, theaforementioned oil absorption amount set at 1000 ml/100 g or less ispreferable from the viewpoint of the handleability of the inorganicfiller at the time of producing the polyolefin microporous membrane.

The proportion of the inorganic filler in the total amount of theinorganic filler and the polyolefin resin is preferably 5% by mass ormore, more preferably 10% by mass or more and furthermore preferably 20%by mass or more, and the upper limit of the aforementioned proportion ispreferably 80% by mass or less, more preferably 60% by mass or less,furthermore preferably 50% by mass or less and particularly preferably40% by mass or less.

The aforementioned proportion set at 5% by mass or more is preferablefrom the viewpoint of forming the polyolefin microporous membrane so asto have a high porosity, the viewpoint of improving the thermalcontraction ratio of the polyolefin microporous membrane at 140° C. inthe transverse direction (widthwise direction, TD direction) of thepolyolefin microporous membrane, and moreover, the viewpoint ofregulating the membrane thickness retention ratio to be high and themembrane thickness reduction ratio to be small in penetration creep. Theaforementioned proportion set at 20% by mass or more is preferable fromthe viewpoint of improving the heat resistance.

On the other hand, the aforementioned proportion set at 80% by mass orless is preferable from the viewpoint of improving the membraneformability at a high stretching magnification and improving thepuncture strength of the polyolefin microporous membrane.

The volume proportion of the inorganic filler in the propylene-basedresin composition is preferably 10% by volume or more, more preferably15% by volume and furthermore preferably 20% by volume or more, and theupper limit of the aforementioned proportion is preferably 80% byvolume.

The specific gravity of the inorganic filler is preferably 1.0 g/cm³ ormore, more preferably 1.2 g/cm³ ore more and furthermore preferably 1.5g/cm³ or more, and the upper limit of the aforementioned specificgravity is preferably 10.0 g/cm³ or less.

The mixing ratio between the propylene copolymer and the inorganicfiller (propylene copolymer)/(inorganic filler) (mass ratio) is 0.1/1 to1.5/1, preferably 0.1/1 to 1.3/1 and furthermore preferably 0.1/1 to1.2/1. The mixing ratio set to fall within the above-described range ispreferable from the viewpoint of suppressing the skin layer formationdue to low molecular weight polymers and polymers low in crystallinity.

The propylene-based resin composition may further include variousadditives.

Examples of such additives include the same additives as listed inEmbodiment 2.

In the laminated polyolefin microporous membrane of Embodiment 4, on atleast one side of the above-described polyolefin microporous membrane,another polyolefin microporous membrane different from theabove-described polyolefin microporous membrane is laminated. Theformation of such a laminated polyolefin microporous membrane ispreferable from the viewpoint of imparting other performances such aslow fuse function. From the viewpoint of the productivity, morepreferable is an aspect of the two-type three-layer structure in whichthe two layers as the surface layers are the same in composition and theintermediate layer is different in composition from these two layers.

As the other polyolefin microporous membrane, heretofore knownmicroporous membranes can be used.

As the method for producing the polyolefin microporous membrane or thelaminated polyolefin microporous membrane, the same production method asin Embodiment 2 can be used.

For the polyolefin microporous membrane or the laminated polyolefinmicroporous membrane (simply abbreviated as the “microporous membrane,”as the case may be) of Embodiment 4, the appropriate numerical ranges ofthe puncture strength, the porosity, the average pore size, the membranethickness, the air permeability, the capacity retention ratio, thehigh-temperature storage property and the short circuit temperature arethe same as in Embodiments 2 and 3.

The microporous membrane is useful as separators for electricity storagedevices, in particular. Usually, an electricity storage device uses theaforementioned microporous membrane for the separator and includes apositive electrode, a negative electrode and an electrolyte.

The electricity storage device can be produced in the same manner as inabove-described Embodiment 2.

The measurement values of the above-described various parameters inEmbodiments 1 to 4 are the values to be measured, unless otherwisespecified, according to the measurement methods in below-describedExamples corresponding to respective Embodiments.

EXAMPLES Embodiment 1

Next, Embodiment 1 is more specifically described with reference toExamples and Comparative Examples; however, Embodiment 1 is not limitedto following Examples unless the gist of Embodiment 1 is exceeded. Thephysical properties in Examples were measured with the followingmethods.

(1) Viscosity Average Molecular Weight (Mv)

The intrinsic viscosity [η] in decalin as solvent at 135° C. wasdetermined on the basis of ASTM-D4020.

The Mv of polyethylene was calculated according to the followingformula:

[η]=6.77×10⁻⁴ Mv ^(0.67)

The Mv of polypropylene was calculated according to the followingformula:

[η]=1.10×10⁻⁴ Mv ^(0.80)

(2) Total Membrane Thickness (μm)

The total membrane thickness was measured at a room temperature of 23°C., with a micro thickness meter (trade name: KBM, manufactured by ToyoSeiki Seisaku-sho, Ltd.).

(3) Surface Layer Thickness (μm)

The surface layer thickness was measured by observing the cross sectionwith a cross section observation method with a scanning electronmicroscope or the like.

(4) Primary Particle Size (nm) of Inorganic Particle

A measurement object was sampled from a laminated separator andsubjected to an observation with a scanning electron microscope at amagnification of 30,000 times, and thus the particle sizes of theinorganic particles were identified in a 3.0 μm×3.0 μm field of view.The term “primary particle size” as referred to herein means theparticle size in the condition that the individual particles areindependently dispersed in a matrix, or when the particles areaggregated, the primary particle size means the size of the smallestaggregate particle of the aggregated particles. An average value of theobserved values at ten different positions was taken as the measuredvalue.

(5) Porosity (%)

From a microporous membrane, a 10 cm×10 cm square sample was cut out,and the volume (cm³) and the mass (g) were determined; from thesedetermined values and the membrane density (g/cm³), the porosity wascalculated according to the following formula:

Porosity (%)=(volume−mass/density of mixed composition)/volume×100

The density of the mixed composition used was a value calculated fromthe densities of the polyolefin resin used and the inorganic particleused, and the mixing ratio between the polyolefin resin and theinorganic particle.

(6) Air Permeability (sec/100 cc)

The air permeability was measured with a Gurley air permeability tester(manufactured by Toyo Seiki Seisaku-sho, Ltd.) according to JIS P-8117.

(7) Puncture Strength (N/20 μm)

A puncture test was performed with a handy compression tester, KES-G5(trade name), manufactured by Kato Tech Co., Ltd., under the conditionsof a needle tip having a curvature radius of 0.5 mm and a puncture speedof 2 mm/sec. The maximum puncture load was taken as the puncturestrength (N). By multiplying this value by 20 (μm)/membrane thickness(μm), the puncture strength (N/20 μm) in terms of the 20-μm membranethickness was calculated.

(8) Shutdown Temperature (° C.)

FIG. 1 schematically shows a shutdown temperature measurement apparatus.Reference sign 1 represents a microporous membrane (a laminatedseparator as a measurement object), reference signs 2A and 2B represent10-μm thick nickel foils, reference signs 3A and 3B represent glassplates. Reference sign 4 represents an electrical resistance measurementapparatus (AG-4311 (trademark), an LCR meter, manufactured by AndoElectric Co., Ltd.) which is connected to the nickel foils 2A and 2B.Reference sign 5 represents a thermocouple which is connected to athermometer 6. Reference sign 7 represents a data collector which isconnected to the electrical resistance measurement apparatus 4 and thethermometer 6. Reference sign 8 represents an oven which is used forheating the microporous membrane.

More specifically, as shown in FIG. 2, the microporous membrane 1 issuperposed on the nickel foil 2A, and longitudinally fixed to the nickelfoil 2A with a “Teflon (registered trademark)” tape (the shaded sectionin the figure). The microporous membrane 1 is impregnated with a 1mol/liter lithium borofluoride solution (solvent: propylenecarbonate/ethylene carbonate/γ-butyl lactone=1/1/2). Onto the nickelfoil 2B, as shown in FIG. 3, a “Teflon (registered trademark)” tape (theshaded section in the figure) was bonded, and then the nickel foil 2Bwas masked with a 15 mm×10 mm window section left unmasked in thecentral section of the foil 2B.

The nickel foil 2A and the nickel foil 2B were superposed on each otherso as to sandwich the microporous membrane 1, and the two-sheets of thenickel foils were sandwiched from the both sides with the glass plates3A and 3B. In this case, the window section of the foil 2B and themicroporous membrane 1 were arranged so as to face each other.

The two sheets of the glass plates were fixed by clipping withcommercially available double clips. The thermocouple 5 was fixed ontothe glass plates with a “Teflon (registered trademark)” tape.

With such an apparatus, the temperature and the electrical resistancewere continuously measured. The temperature was increased from 25° C. to200° C. at a rate of 2° C./min, and the electrical resistance wasmeasured with an alternating current of 1 V and 1 kHz. The shutdowntemperature was defined as the temperature where the electricalresistance of the microporous membrane reached 10³Ω.

(9) Soldering Test (mm²)

A soldering iron of 1 mm in diameter was arranged so as to beperpendicular to the microporous membrane fixed to a frame. Thetemperature of the soldering iron was set at 300° C. or 400° C. When thetemperature of the soldering iron was stabilized, the soldering iron wasallowed to move downward at a rate of 10 mm/sec, and was allowed topuncture the microporous membrane for 3 seconds, and then was allowed tomove upward. The area of the thus formed hole was observed with anoptical microscope and was subjected to an image processing to measurethe area.

(10) Capacity Retention Ratio (%)

a. Preparation of Positive Electrode

A slurry was prepared by dispersing 92.2% by mass of a lithium cobaltcomposite oxide (LiCoO₂) as a positive electrode active material, 2.3%by mass of a scale-like graphite and 2.3% by mass of acetylene black,both being conductive, and 3.2% by mass of polyvinylidene fluoride(PVDF) as a binder in N-methylpyrrolidone (NMP). The slurry was appliedonto one side of a 20-μm thick aluminum foil to be a positive electrodecurrent collector with a die coater, dried at 130° C. for 3 minutes, andthen compression molded with a roll press. In this case, the coatingamount of the active material of the positive electrode was regulated tobe 250 g/m² and the active material bulk density of the positiveelectrode was regulated to be 3.00 g/cm³.

b. Preparation of Negative Electrode

A slurry was prepared by dispersing 96.9% by mass of an artificialgraphite as a negative electrode active material, and 1.4% by mass of anammonium salt of carboxymethyl cellulose and 1.7% by mass of astyrene-butadiene copolymer latex, both being binders, in purifiedwater. The slurry was applied onto one side of a 12-μm thick copper foilto be a negative electrode current collector with a die coater, dried at120° C. for 3 minutes, and then compression molded with a roll press. Inthis case, the coating amount of the active material of the negativeelectrode was regulated to be 106 g/m² and the active material bulkdensity of the negative electrode was regulated to be 1.35 g/cm².

c. Preparation of Nonaqueous Electrolyte

A nonaqueous electrolyte was prepared by dissolving LiPF₆ as a solute ina mixed solvent of ethylene carbonate:ethyl methyl carbonate=1:2 (volumeratio) so as for the concentration to be 1.0 mol/L.

d. Assembly of Battery

The separator was cut out as a 30-mm φ disc, and the positive electrodeand the negative electrode were each cut out as a 16-mm φ disc. Thenegative electrode, the separator and the positive electrode weresuperposed in this order so as for the active material sides of thepositive electrode and the negative electrode to face each other, andthe resulting superposed assembly was housed in a stainless steel vesselwith a lid. The vessel and the lid were insulated, and the vessel wasbrought into contact with the copper foil of the negative electrode andthe lid was brought into contact with the aluminum foil of the positiveelectrode. The above-described nonaqueous electrolyte was injected intothe vessel and the vessel was hermetically sealed. The assembled batterywas allowed to stand at room temperature for 1 day, and then the firstcharge of the battery subsequent to the assembly of the battery wasperformed for a total time of 8 hours in such a way that the battery wascharged in an atmosphere of 25° C. with a current value of 2.0 mA (0.33C) until the battery voltage reached 4.2 V; after the attainment of 4.2V, while the battery voltage was being regulated to hold 4.2 V, thecurrent value was decreased starting from 2.0 mA. Successively, thebattery was discharged down to a battery voltage of 3.0 V with a currentvalue of 2.0 mA (0.33 C).

e. Capacity Retention Ratio (%)

In the atmosphere of 60° C., 100 cycles of charge and discharge wereperformed. The charge of the battery was performed for a total time of 3hours in such a way that the battery was charged with a current value of6.0 mA (1.0 C) until the battery voltage reached 4.2 V; after theattainment of 4.2 V, while the battery voltage was being regulated tohold 4.2 V, the current value was decreased starting from 6.0 mA. Thedischarge of the battery was performed with a current value of 6.0 mA(1.0 C) until the battery voltage reached 3.0 V. From the dischargecapacity of the 100th cycle and the discharge capacity of the firstcycle, the capacity retention ratio was calculated. The cases where thecapacity retention ratio was high were evaluated to have a satisfactorycycle property.

Example 1-1

The raw material mixture for a first polyolefin microporous layer wasprepared by preliminarily mixing a mixture with a super mixer. Themixture includes: SiO₂ having a primary particle size of 15 nm, “DM10C”(trademark, manufactured by Tokuyama Corp., hydrophobized withdimethyldichlorosilane) in an amount of 24.0 parts by mass (60% by massas the inorganic particle concentration); an ultrahigh molecular weightpolyethylene having a viscosity average molecular weight (Mv) of2,000,000, “UH850” (trademark, manufactured by Asahi Kasei ChemicalsCorp.) in an amount of 6.4 parts by mass (16% by mass as the proportionin the total amount of the polyolefin resin and the inorganic particle);and a homopolypropylene having a Mv of 400,000, “H-100M” (manufacturedby Prime Polymer Co., Ltd.) in an amount of 9.6 parts by mass (24% bymass as the proportion in the total amount of the polyolefin resin andthe inorganic particle); and further, 28.8 parts by mass of a liquidparaffin “Smoil P-350P” (trademark, Matsumura Oil Research Corp.) addedas a plasticizer and 0.3 part by mass ofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]added as an antioxidant.

The raw material mixture for a second polyolefin microporous layer wasprepared as a mixture including: a high-density polyethylene “UH650”(trademark, manufactured by Asahi Kasei Chemicals Corp.) having a Mv of700,000 in an amount of 20 parts by mass (50% by mass in the totalamount of the polyolefin resin); and a high-density polyethylene “SH800”(trademark, manufactured by Asahi Kasei Chemicals Corp.) having a Mv of270,000 in an amount of 20 parts by mass (50% by mass in the totalamount of the polyolefin resin); and further, 0.3 part by mass ofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]added as an antioxidant.

The resulting raw material mixtures were fed with feeders to the feedinlets of two corotation double screw extruders, respectively. In eachof the raw material mixtures, a liquid paraffin was side fed to thecylinder of the concerned double screw extruder in such a way that theproportion of the plasticizer in the whole mixture to be melt-kneadedand extruded was regulated to be 60% by mass. The melt-kneadingconditions in the extruders were such that the raw material mixture forthe first microporous layer was melt-kneaded at a temperature set at200° C., a screw rotation number of 100 rpm and a discharge rate of 5kg/h, and the raw material mixture for the second microporous layer wasmelt-kneaded at a temperature set at 200° C., a screw rotation number of120 rpm and a discharge rate of 16 kg/h.

Successively, the melt-kneaded mixtures were respectively passed throughgear pumps, ducts and a T-die capable of performing two-type three-layercoextrusion, all set at a temperature of 220° C., extruded onto a rollhaving a surface temperature controlled at 30° C., and cooled with aroll having a surface temperature of 25° C., to yield a 1200-μm thicksheet-like composition in which the first layers formed of the rawmaterial for the first microporous layer were the surface layers. Next,the sheet-like composition was successively led to a simultaneousbiaxial tenter and was simultaneously biaxially stretched with alongitudinal magnification of 7 times and a transverse magnification of7 times. The set temperature of the simultaneous biaxial tenter was 123°C. Then, the sheet-like composition was introduced into an extractionvessel, and sufficiently immersed in methylene chloride, and thus theliquid paraffin was extracted and removed. Then, the methylene chloridein the sheet-like composition was dried. Further, the sheet-likecomposition was led to a transverse tenter, stretched in TD with amagnification of 1.4 times and relaxed so as to have a magnification of1.2 times in TD at the final outlet and then taken up (in the concernedtables, described as “1.4-1.2”). The set temperature of the TDstretching section was 120° C. and the set temperature of the relaxationsection was 125° C. (in the concerned tables, described as “120-125”).The properties of the thus obtained laminated separator are shown inTable 1.

Examples 1-2 to 1-17 and Comparative Example 1-2

Laminated separators were obtained in the same manner as in Example 1-1except for the conditions described in Tables 1 and 2. The results areshown in Tables 1 and 2.

Example 1-18

A laminated separator was obtained in the same manner as in Example 1-1except that the raw material for the first polyolefin microporous layerwas prepared by using SiO₂ having a primary particle size of 15 nm in anamount of 24.0 parts by mass, an ultra high molecular weightpolyethylene having a viscosity average molecular weight (Mv) of 270,000in an amount of 5.1 parts by mass, a polyethylene having a viscosityaverage molecular weight (Mv) of 1,000 in an amount of 1.3 parts by massand a homopolypropylene having a Mv of 400,000 in an amount of 9.6 partsby mass.

The obtained laminated separator was excellent in the shutdown property.

The origins of the respective raw materials are as follows.

[Inorganic Particle]

SiO₂: Silica “DM10C” (trademark, manufactured by Tokuyama Corp.,hydrophobized with dimethyldichlorosilane) having an average primaryparticle size of 15 nm

Al₂O₃: Various commercially available aluminas having an average primaryparticle size of 13 nm to 100 nm

[PP]

Mv 400,000: Homopropylene “H-100M” (manufactured by Prime Polymer Co.,Ltd.) having a Mv of 400,000

[PE]

Mv 2,000,000: Ultra high molecular weight polyethylene “UH850”(trademark, manufactured by Asahi Kasei Chemicals Corp.) having a Mv of2,000,000

Mv 700,000: High-density polyethylene “UH650” (trademark, manufacturedby Asahi Kasei Chemicals Corp.) having a Mv of 700,000

Mv 270,000: High-density polyethylene “SH800” (trademark, manufacturedby Asahi Kasei Chemicals Corp.) having a Mv of 270,000

Comparative Example 1-1

A raw material mixture was prepared by preliminarily mixing a mixturewith a super mixer. The mixture includes: silica having an averageprimary particle size of 15 nm, “DM10C” (trademark, manufactured byTokuyama Corp., hydrophobized with dimethyldichlorosilane) in an amountof 24.0 parts by mass; an ultrahigh molecular weight polyethylene havinga viscosity average molecular weight (Mv) of 2,000,000, “UH850”(trademark, manufactured by Asahi Kasei Chemicals Corp.) in an amount of6.4 parts by mass; and a homopolypropylene having a Mv of 400,000,“H-100M” (manufactured by Prime Polymer Co., Ltd.) in an amount of 9.6parts by mass; and further, 28.8 parts by mass of a liquid paraffin“Smoil P-350P” (trademark, manufactured by Matsumura Oil Research Corp.)added as a plasticizer and 0.3 part by mass ofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]added as an antioxidant.

The resulting raw material mixture was fed with a feeder to the feedinlet of a corotation double screw extruder. A liquid paraffin was sidefed to the cylinder of the concerned double screw extruder in such a waythat the proportion of the plasticizer in the whole mixture to bemelt-kneaded and extruded was regulated to be 60% by mass. Themelt-kneading conditions in the extruder were such that the settemperature was 200° C., a screw rotation number was 150 rpm and adischarge rate was 20 kg/h. Successively, the melt-kneaded mixture waspassed through a gear pump, a duct and a monolayer T-die, all set at atemperature of 220° C., extruded onto a roll having a surfacetemperature controlled at 30° C., and cooled with a roll having asurface temperature controlled at 25° C., to yield a 1200-μm thicksheet-like composition. Next, the sheet-like composition wassuccessively led to a simultaneous biaxial tenter and was simultaneouslybiaxially stretched with a longitudinal magnification of 7 times and atransverse magnification of 7 times. The set temperature of thesimultaneous biaxial tenter was 123° C. Then, the sheet-like compositionwas introduced into an extraction vessel, and sufficiently immersed inmethylene chloride, and thus the liquid paraffin was extracted andremoved. Then, the methylene chloride in the sheet-like composition wasdried. Further, the sheet-like composition was led to a transversetenter, stretched in the transverse direction with a magnification of1.4 times and relaxed so as to have a magnification of 1.2 times at thefinal outlet and then taken up. The set temperature of the transversestretching section was 120° C. and the set temperature of the relaxationsection was 125° C. The properties of the thus obtained microporousmembrane are shown in Table 2.

TABLE 1 Example 1-1 Example 1-2 Example 1-3 Example 1-4 Example 1-5Structure First First First First First layer/second layer/secondlayer/second layer/second layer/second layer/first layer/firstlayer/first layer/first layer/first layer layer layer layer layer Firstlayer Type of inorganic particle SiO2 Al2O3 Al2O3 Al2O3 SiO2 compositionPrimary particle size of nm 15 13 50 80 15 inorganic particle PP (Mv) inunits of 40 40 40 40 40 10000 PE (Mv) in units of 200 200 200 200 20010000 Inorganic particle mass % 60 60 60 60 80 PP mass % 24 24 24 24 12Propylene-ethylene mass % — — — — — copolymer PE mass % 16 16 16 16 8 PPcontent in first layer mass % 60 60 60 60 80 (ratio of PP to PO) Secondlayer Type of inorganic particle — — — — — composition Primary particlesize of nm — — — — — inorganic particle PP (Mv) in units of — — — — —10000 PE1 (Mv) in units of 70 70 70 70 70 10000 PE2 (Mv) in units of 2727 27 27 27 10000 Inorganic particle mass % 0 0 0 0 0 PP mass % 0 0 0 00 PE1 mass % 50 50 50 50 50 PE2 mass % 50 50 50 50 50 ExtrusionExtrusion rate kg/h 5 5 5 5 5 (first layer) Plasticizer proportion mass% 60 60 60 60 60 Rotation number rpm 100 100 100 100 100 Temperature °C. 200 200 200 200 200 Extrusion Extrusion rate kg/h 16 16 16 16 16(second layer) Plasticizer proportion mass % 60 60 60 60 60 Rotationnumber rpm 120 120 120 120 120 Temperature ° C. 200 200 200 200 200Biaxial tenter Temperature ° C. 123 123 123 123 123 Magnification times7 × 7 7 × 7 7 × 7 7 × 7 7 × 7 Relaxation Temperature ° C. 120-125120-125 120-125 120-125 120-125 Magnification times 1.4-1.2 1.4-1.21.4-1.2 1.4-1.2 1.4-1.2 Total μm 18 18 18 18 18 membrane thicknessSurface layer μm 2 (both of 2 2 (both of 2 2 (both of 2 2 (both of 2 2(both of 2 thickness layers) layers) layers) layers) layers) Porosity %57 55 58 60 61 Air sec/100 cc 170 188 159 133 131 permeability PunctureN/20 μm 4.5 4.5 4.4 4.2 4.3 strength Shutdown ° C. 141 141 141 141 141temperature Soldering test 300° C. mm2 3.3 3.5 3.9 4.4 3.1 400° C. mm26.1 6.3 7.0 7.5 5.8 Capacity % 87 87 86 84 88 retention ratio Example 1-Example 1-6 Example 1-7 Example 1-8 Example 1-9 10 Structure First FirstFirst First First layer/second layer/second layer/second layer/secondlayer/second layer/first layer/first layer/first layer/first layer/firstlayer layer layer layer layer First layer Type of inorganic particleSiO2 SiO2 SiO2 SiO2 SiO2 composition Primary particle size of nm 15 1515 15 15 inorganic particle PP (Mv) in units of 40 40 40 40 40 10000 PE(Mv) in units of 200 200 200 200 200 10000 Inorganic particle mass % 4020 60 60 60 PP mass % 36 48 32 16 24 Propylene-ethylene mass % — — — — —copolymer PE mass % 24 32 8 24 16 PP content in first layer mass % 40 2060 60 60 (ratio of PP to PO) Second layer Type of inorganic particle — —— — — composition Primary particle size of nm — — — — — inorganicparticle PP (Mv) in units of — — — — — 10000 PE1 (Mv) in units of 70 7070 70 70 10000 PE2 (Mv) in units of 27 27 27 27 27 10000 Inorganicparticle mass % 0 0 0 0 0 PP mass % 0 0 0 0 0 PE1 mass % 50 50 50 50 50PE2 mass % 50 50 50 50 50 Extrusion Extrusion rate kg/h 5 5 5 5 10(first layer) Plasticizer proportion mass % 60 60 60 60 60 Rotationnumber rpm 100 100 100 100 120 Temperature ° C. 200 200 200 200 200Extrusion Extrusion rate kg/h 16 16 16 16 11 (second layer) Plasticizerproportion mass % 60 60 60 60 60 Rotation number rpm 120 120 120 120 100Temperature ° C. 200 200 200 200 200 Biaxial tenter Temperature ° C. 123123 123 123 123 Magnification times 7 × 7 7 × 7 7 × 7 7 × 7 7 × 7Relaxation Temperature ° C. 120-125 120-125 120-125 120-125 120-125Magnification times 1.4-1.2 1.4-1.2 1.4-1.2 1.4-1.2 1.4-1.2 Total μm 1818 18 18 18 membrane thickness Surface layer μm 2 (both of 2 2 (both of2 2 (both of 2 2 (both of 2 4 (both of 2 thickness layers) layers)layers) layers) layers) Porosity % 53 49 56 58 60 Air sec/100 cc 208 234183 165 142 permeability Puncture N/20 μm 4.5 4.5 4.5 4.5 4.4 strengthShutdown ° C. 140 139 141 141 144 temperature Soldering test 300° C. mm24.5 4.8 3.2 3.8 3.3 400° C. mm2 7.4 7.9 5.9 6.8 6.0 Capacity % 84 82 8884 87 retention ratio

TABLE 2 Example 1- Example 1- Example 1- Example 1- Example 1- 11 12 1314 15 Structure Second First First Second First layer/first layer/secondlayer/second layer/first layer/second layer/second layer/firstlayer/first layer/second layer/first layer layer layer layer layer Firstlayer Type of inorganic particle SiO2 SiO2 SiO2 SiO2 SiO2 compositionPrimary particle size of inorganic nm 15 15 15 15 15 particle PP (Mv) inunits of 40 40 40 40 40 10000 PE (Mv) in units of 200 200 200 27 2710000 Inorganic particle mass % 60 60 60 30 30 PP mass % 24 24 24 49 28Propylene-ethylene copolymer mass % — — — — — PE mass % 16 16 16 21 42PP content in first layer (ratio of PP mass % 60 60 60 70 40 to PO)Second layer Type of inorganic particle — — SiO2 — — composition Primaryparticle size of inorganic nm — — 15 — — particle PP (Mv) in units of —40 — — — 10000 PE1 (Mv) in units of 70 70 70 70 70 10000 PE2 (Mv) inunits of 27 27 27 27 27 10000 Inorganic particle mass % 0 0 15 0 0 PPmass % 0 15 0 5 20 PE1 mass % 50 42.5 42.5 47.5 40 PE2 mass % 50 42.542.5 47.5 40 Extrusion (first Extrusion rate kg/h 14 5 5 5 5 layer)Plasticizer proportion mass % 60 60 60 60 60 Rotation number rpm 140 100100 100 100 Temperature ° C. 200 200 200 200 200 Extrusion Extrusionrate kg/h 7 16 16 16 16 (second layer) Plasticizer proportion mass % 6060 60 60 60 Rotation number rpm 80 120 120 120 120 Temperature ° C. 200200 200 200 200 Biaxial tenter Temperature ° C. 123 123 123 123 123Magnification times 7 × 7 7 × 7 7 × 7 7 × 7 7 × 7 Relaxation Temperature° C. 120-125 120-125 120-125 120-125 120-125 Magnification times 1.4-1.21.4-1.2 1.4-1.2 1.4-1.2 1.4-1.2 Total μm 18 18 18 18 18 membranethickness Surface layer μm 3 (both of 2 2 (both of 2 2 (both of 2 2(both of 2 2 (both of 2 thickness μm layers) layers) layers) layers)layers) Porosity % 62 58 55 52 56 Air sec/100 cc 127 179 161 220 150permeability Puncture N/20 μm 3.3 4.5 4.5 3.5 3.5 strength Shutdown ° C.143 143 143 141 141 temperature Soldering test 300° C. mm2 4.2 3.3 3.14.2 4.4 400° C. mm2 7.2 6.0 5.9 7.0 7.4 Capacity % 84 87 87 84 83retention ratio Example 1- Example 1- Comparative Comparative 16 17Example 1-1 Example 1-2 Structure Firstlayer/second Second layer/firstMonolayer First layer/second layer/first layer/second layer/first layerlayer layer First layer Type of inorganic particle SiO2 SiO2 SiO2 Al2O3composition Primary particle size of inorganic nm 15 15 15 100 particlePP (Mv) in units of 40 40 40 40 10000 PE (Mv) in units of 27 27 200 20010000 Inorganic particle mass % 30 30 60 60 PP mass % 34 34 24 24Propylene-ethylene copolymer mass % 15 15 — — PE mass % 21 21 16 16 PPcontent in first layer (ratio of PP mass % 70 70 60 60 to PO) Secondlayer Type of inorganic particle — — — — composition Primary particlesize of inorganic nm — — — — particle PP (Mv) in units of — — — — 10000PE1 (Mv) in units of 70 70 — 70 10000 PE2 (Mv) in units of 27 27 — 2710000 Inorganic particle mass % 0 0 — 0 PP mass % 5 5 — 0 PE1 mass %47.5 47.5 — 50 PE2 mass % 47.5 47.5 — 50 Extrusion (first Extrusion ratekg/h 5 5 — 5 layer) Plasticizer proportion mass % 60 60 — 60 Rotationnumber rpm 100 100 — 100 Temperature ° C. 200 200 — 200 ExtrusionExtrusion rate kg/h 16 16 16 5 (second layer) Plasticizer proportionmass % 60 60 60 60 Rotation number rpm 120 120 120 100 Temperature ° C.200 200 200 200 Biaxial tenter Temperature ° C. 123 123 123 123Magnification times 7 × 7 7 × 7 7 × 7 7 × 7 Relaxation Temperature ° C.120-125 120-125 120-125 120-125 Magnification times 1.4-1.2 1.4-1.21.4-1.2 1.4-1.2 Total μm 18 18 18 18 membrane thickness Surface layer μm2 (both of 2 2 (both of 2 18 2 (both of 2 thickness μm layers) layers)(Monolayer) layers) Porosity % 53 53 66 63 Air sec/100 cc 210 230 82 106permeability Puncture N/20 μm 3.4 3.4 2.9 4 strength Shutdown ° C. 140140 — 141 temperature Soldering test 300° C. mm2 4.6 4.6 5.4 5.2 400° C.mm2 7.4 7.4 8.5 8.3 Capacity % 85 86 78 79 retention ratio

From the results shown in Tables 1 and 2, the following contents can berecognized.

(1) As can be seen from a comparison between Examples 1-1 to 1-4 andComparative Examples 1-1 and 1-2, the laminated separators of Embodiment1 including inorganic particles having primary particle sizes fallingwithin a specific range and having a laminated structure provides asatisfactory compatibility between the heat resistance, the cycleproperty and the shutdown property with a satisfactory balancetherebetween, in contrast to an embodiment in which the primary particlesize deviates from the specific range or an embodiment which has nolaminated structure.

(2) As can be seen from the results of Example 1-1 and Examples 1-5 to1-7, the effect of establishing the compatibility between the heatresistance, the cycle property and the shutdown property is realizedover a wide range of the inorganic particle content.

(3) As can be seen from the results of Example 1-1 and Examples 1-8 and1-9, the effect of establishing the compatibility between the heatresistance, the cycle property and the shutdown property is realizedover a wide range of the PP content.

(4) As can be seen from the results of Example 1-1 and Examples 1-10 to1-13, the effect of establishing the compatibility between the heatresistance, the cycle property and the shutdown property is realized forvarious surface layer thicknesses and for various layer structures.

Embodiment 2

Next, Embodiment 2 is more specifically described with reference toExamples and Comparative Examples; however, Embodiment 2 is not limitedto following Examples unless the gist of Embodiment 2 is exceeded. Thephysical properties in Examples were measured with the followingmethods.

(1) Viscosity Average Molecular Weight (Mv)

The intrinsic viscosity [η] in decalin as solvent at 135° C. wasdetermined on the basis of ASTM-D4020.

The Mv of polyethylene was calculated according to the followingformula:

[η]=0.00068×Mv ^(0.67)

The Mv of polypropylene was calculated according to the followingformula:

[η]=1.10×Mv ^(0.80)

The Mv of a layer was calculated by using the formula for polyethylene.

(2) Membrane Thickness (μm)

The membrane thickness was measured at a room temperature of 23±2° C.,with a micro thickness meter (trade name: KBM, manufactured by ToyoSeiki Seisaku-sho, Ltd.).

(3) Porosity (%)

From a microporous membrane, a 10 cm×10 cm square sample was cut out,and the volume (cm³) and the mass (g) were determined; from thesedetermined values and the membrane density (g/cm3), the porosity wascalculated according to the following formula:

Porosity (%)=(volume−mass/density of mixed composition)/volume×100

The density of the mixed composition used was a value calculated fromthe densities of the polyolefin resin used and the inorganic fillerused, and the mixing ratio between the polyolefin resin and theinorganic filler.

(4) Air Permeability (sec/100 cc)

The air permeability was measured with a Gurley air permeability tester(manufactured by Toyo Seiki Seisaku-sho, Ltd.) according to JIS P-8117.

(5) Puncture Strength (g)

A puncture test was performed with a handy compression tester, KES-G5(trademark), manufactured by Kato Tech Co., Ltd., under the conditionsof a needle tip having a curvature radius of 0.5 mm and a puncture speedof 2 mm/sec. The maximum puncture load was taken as the puncturestrength (g).

(6) Electrolyte Impregnation Property

As an electrolyte, a 1 mol/L lithium hexafluorophosphate (solvent:propylene carbonate) was used. In a glove box, the electrolyte wasdropwise placed on a microporous membrane. The case where 80% or more ofthe area in which the electrolyte droplet and the microporous membranewere brought into contact with each other was transparent after anelapsed time of 30 seconds was evaluated that the impregnation propertywas satisfactory (◯), and the case where less than 80% of theaforementioned area was transparent after an elapsed time of 30 secondswas evaluated that the impregnation property was insufficient (x).

(7) Cycle Property (%/100 times)

An electrolyte was prepared by dissolving 1 M of LiPF₆ in a mixedsolvent composed of ethylene carbonate (EC):methylene carbonate(MEC)=1:2 (weight ratio). A carbon electrode was used for the negativeelectrode, and LiCoO₂ was used for the positive electrode. As aseparator, a microporous membrane of a measurement sample was used.Thus, a lithium ion battery was assembled. A cycle test was performed inwhich the operation of charging the battery to 4.2 V and thendischarging the battery was repeated at 25° C. 100 times. The batterycapacity change after the cycle test was examined.

(8) Shutdown Temperature (° C.) and Short-Circuit Temperature (° C.)

FIG. 1 schematically shows a shutdown temperature measurement apparatus.Reference sign 1 represents a microporous membrane (a laminatedseparator as a measurement object), reference sign 2A and 2B represent10-μm thick nickel foils, and reference signs 3A and 3B represent glassplates. Reference sign 4 represents an electrical resistance measurementapparatus (AG-4311 (trademark), an LCR meter, manufactured by AndoElectric Co., Ltd.) which is connected to the nickel foils 2A and 2B.Reference sign 5 represents a thermocouple which is connected to athermometer 6. Reference sign 7 represents a data collector which isconnected to the electrical resistance measurement apparatus 4 and thethermometer 6. Reference sign 8 represents an oven which is used forheating the microporous membrane.

More specifically, as shown in FIG. 2, the microporous membrane 1 wassuperposed on the nickel foil 2A, and longitudinally fixed to the nickelfoil 2A with a “Teflon (registered trademark)” tape (the shaded sectionin the figure). The microporous membrane 1 was impregnated with a 1mol/liter lithium borofluoride solution (solvent: propylenecarbonate/ethylene carbonate/γ-butyl lactone=1/1/2). Onto the nickelfoil 2B, as shown in FIG. 3, a “Teflon (registered trademark)” tape (theshaded section in the figure) was bonded, and thus the nickel foil 2Bwas masked with a 15 mm×10 mm window section left unmasked in thecentral section of the foil 2B.

The nickel foil 2A and the nickel foil 2B were superposed on each otherso as to sandwich the microporous membrane 1, and the two-sheets of thenickel foils were sandwiched from the both sides with the glass plates3A and 3B. In this case, the window section of the foil 2B and themicroporous membrane 1 were arranged so as to face each other.

The two sheets of the glass plates were fixed by clipping withcommercially available double clips. The thermocouple 5 was fixed ontothe glass plates with a “Teflon (registered trademark)” tape.

With such an apparatus, the temperature and the electrical resistancewere continuously measured. The temperature was increased from 25° C. to200° C. at a rate of 2° C./min, and the electrical resistance wasmeasured with an alternating current of 1 V and 1 kHz. The shutdowntemperature was defined as the temperature where the electricalresistance of the microporous membrane reached 10³Ω. The short-circuittemperature was defined as the temperature where after the membraneunderwent shutdown and the pores reached a blocked condition, theimpedance again reached a value smaller than 10³Ω.

(9) Ethylene Content in Polypropylene-Ethylene Copolymer

From a ¹³C-NMR spectrum measured under the following conditions, theethylene content was determined on the basis of the report published byKakugo et al. (Macromolecules 1982, 15, 1150-1152.). In a 10-mmφ testtube, about 200 mg of propylene-ethylene block copolymer was uniformlydissolved in 3 ml of ortho-dichlorobenzene to prepare a sample, and ameasurement was performed under the following conditions.

Measurement temperature: 135° C.

Pulse repetition time: 10 seconds

Pulse width: 45°

Accumulation number: 2500

(10) Plasticizer Oil Absorption Amount of Inorganic Filler

The measurement was performed as follows with a FRONTEX 5410 plasticizeroil absorption meter. In the meter, 5 g of an inorganic filler wasplaced, and a plasticizer (paraffin oil) was added dropwise withkneading. The torque during the kneading was increased and thendecreased; accordingly, the addition amount (ml) of the plasticizer atthe time when the torque was decreased to 70% of the maximum torque wasmeasured; the plasticizer oil absorption amount was calculated from thisaddition amount (ml) and the weight (g) of the inorganic filleraccording to the following formula.

Plasticizer oil absorption amount (mL/100 g)=addition amount ofplasticizer/weight of inorganic filler×100

Example 2-1

A raw material mixture was prepared by mixing 21.2 parts by mass (78% bymass) of a polypropylene (melting point: 163° C.) having a viscosityaverage molecular weight of 400,000, 0.4 part by mass (2% by mass) of apropylene-ethylene copolymer (melting point: 160° C.) having a viscosityaverage molecular weight of 250,000 and an ethylene content of 3% bymass, 5.4 parts by mass (20% by mass) of a high-density polyethylene(melting point: 137° C.) having a viscosity average molecular weight of250,000, 0.3 part by mass of bis(p-ethylbenzylidene) sorbitol as anucleating agent, 0.2 part by mass oftetrakis-[methylene-(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionate]methane as an antioxidant and 63 parts by mass (70% by mass)of a paraffin oil (P350P, manufactured by Matsumura Oil Research Corp.).The resulting raw material mixture was kneaded with a batch-type meltkneader (Laboplastomill, manufactured by Toyo Seiki Seisaku-sho, Ltd.)at 200° C. at 50 rpm for 10 minutes. The resulting kneaded mixture wasmolded at 5 MPa with a hot press set at 200° C., and heat treated as itwas for 3 minutes, and then cooled at 5 MPa with a water-cooling presscontrolled at 25° C. to form a 500-μm thick sheet. The sheet wasstretched 4 times×4 times at 120° C. with a simultaneous biaxialstretching machine (manufactured by Toyo Seiki Seisaku-sho, Ltd.), thenthe paraffin oil was extracted and removed with methylene chloride, andthe sheet was dried. The physical properties of the obtainedpolyethylene microporous membrane are shown in Table 3.

Examples 2-2 to 2-5 and Comparative Examples 2-1 to 2-3

Polyethylene microporous membranes were obtained in the same manner asin Example 2-1 except for the conditions shown in Table 3. The physicalproperties of the obtained polyethylene microporous membranes are shownin Table 3.

The oil absorption amount of the silica used as the inorganic filler was200 mL/100 g.

Example 2-6

The raw material mixture for the surface layer was prepared by stirringwith a mixer 31.4 parts by mass of a polypropylene (melting point: 163°C.) having a viscosity average molecular weight of 400,000, 0.6 part bymass of a propylene-ethylene copolymer (melting point: 160° C.) having aviscosity average molecular weight of 250,000 and an ethylene content of3% by mass, 8.0 parts by mass of a high-density polyethylene (meltingpoint: 137° C.) having a viscosity average molecular weight of 250,000,0.3 part by mass of bis(p-ethylbenzylidene) sorbitol as a nucleatingagent, 0.2 part by mass oftetrakis-[methylene-(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionate]methane as an antioxidant and 9.6 parts by mass of a liquidparaffin (P350P, manufactured by Matsumura Oil Research Corp.) as aplasticizer.

The raw material mixture for the intermediate layer was prepared bymixing 14.25 parts by mass of a high-density polyethylene having aviscosity average molecular weight of 250,000, 14.25 parts by mass of ahigh-density polyethylene having a viscosity average molecular weight of700,000, 1.5 parts by mass of a polypropylene having a viscosity averagemolecular weight of 400,000 and 0.2 part by mass oftetrakis-[methylene-(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionate]methaneas an antioxidant.

The resulting raw material mixtures were fed through feeders to twodouble screw extruders each having a diameter of 25 mm and with L/D=48.Further, 65 parts by mass of a liquid paraffin and 70 parts by mass of aliquid paraffin were added to the raw material mixture for the surfacelayer and the raw material mixture for the intermediate layer,respectively, in such a way that, for each of these raw materialmixtures, liquid paraffin was injected into the concerned double screwextruder by side feeding. The extrusion rates for the both surfacelayers and the intermediate layer were regulated to be 4 kg per 1 hourand 16 kg per 1 hour, respectively. After kneading under the conditionsof 200° C. and 200 rpm, the kneaded raw material mixtures were extrudedfrom a T-die, fixed to the end of each of the extruders, capable ofperforming coextrusion (two-type three-layer) under a condition of 220°C. Immediately, the extruded product was extruded to a roll having asurface temperature controlled at 70° C., and further cooled with a rollhaving a surface temperature controlled at 25° C., and thus, a 1.4-mmthick sheet was formed. The sheet was stretched 7 times×7 times under acondition of 123° C. with a simultaneous biaxial stretching machine,then immersed in methylene chloride to extract and remove the liquidparaffin, and then dried. The dried sheet was stretched with a tenterstretching machine under a condition of 119° C. in the transversedirection with a magnification of 1.4 times. Then, the stretched sheetwas heat treated at 124° C. to be relaxed by 21% in the transversedirection, and thus a microporous membrane having a two-type three-layerstructure in which the two surface layers were the same in composition,and the intermediate layer was different in composition from the surfacelayers was obtained. The physical properties of the obtained microporousmembrane are shown in Table 4.

Examples 2-7 to 2-15 and Comparative Examples 2-4 and 2-5

Polyethylene microporous membranes were obtained in the same manner asin Example 2-6 except for the conditions shown in Table 4. The physicalproperties of the obtained polyethylene microporous membranes are shownin Table 4.

Example 2-16

A microporous membrane having a two-type three-layer structure wasobtained in the same manner as in Example 2-6 except that the rawmaterial for the first polyolefin microporous membrane had a compositionincluding 24.4 parts by mass of a polypropylene (melting point: 163° C.)having a viscosity average molecular weight of 400,000, 1.2 parts bymass of a propylene-ethylene copolymer (melting point: 160° C.) having aviscosity average molecular weight of 250,000 and an ethylene content of3% by mass, 5.9 parts by mass of a high-density polyethylene (meltingpoint: 137° C.) having a viscosity average molecular weight of 250,000,0.5 part by mass of a polyethylene (melting point: 120° C.) having aviscosity average molecular weight of 1000 and 8 parts by mass of silicahaving an average primary particle size of 15 nm.

The obtained laminated separator was excellent in the shutdown property.

The oil absorption amount of the silica used as the inorganic filler was200 mL/100 g.

TABLE 3 Example Example Example Example Example Comparative ComparativeComparative Sample 2-1 2-2 2-3 2-4 2-5 Example 2-1 Example 2-2 Example2-3 First layer Polypropylene (A) Mv (in units of 10000) 40 40 40 40 4040 40 40 Melting point (° C.) 163 163 163 163 163 163 163 163 wt % 78 7856 61 45 80 78 64 Propylene-ethylene copolymer Mv (in units of 10000) 2525 25 25 25 — 25 — Melting point (° C.) 160 160 160 160 160 — 160 —Ethylene content (wt %) 3 10 3 3 3 — 20 — wt % 2 2 24 3 19 0 2 0High-density polyethylene Mv (in units of 10000) 25 25 25 25 25 25 25 25Melting point (° C.) 137 137 137 137 137 137 137 137 wt % 20 20 20 16 1620 20 16 Silica wt % 0 0 0 20 20 0 0 20 Polyolefin resin concentration30 30 30 24 24 30 30 24 (wt %)*⁾ Second Polypropylene layer Mv (in unitsof 10000) (B) Melting point (° C.) wt % High-density polyethylene Mv (inunits of 10000) Melting point (° C.) wt % Mv (in units of 10000) Meltingpoint (° C.) wt % Polyolefin resin concentration (wt %)*⁾ ProductionLayer structure A A A A A A A A conditions Kneading temperature (° C.)200 200 200 200 200 200 200 200 First stretching Stretching temperature(° C.) 120 120 120 120 120 123 123 123 Stretching magnification 4 × 4 4× 4 4 × 4 4 × 4 4 × 4 7 × 7 7 × 7 7 × 7 Second stretching (times)Stretching temperature (° C.) — — — — — 119 119 119 Stretchingmagnification (times) — — — — — 1.4 1.4 1.4 Physical Thickness ratio(A/B/A) — — — — — — — — properties Membrane thickness (μm) 18 18 18 1818 18 18 18 of Porosity (%) 64 62 64 64 62 59 59 61 membrane Airpermeability (s/100 cc) 140 190 150 115 130 286 300 270 Puncturestrength (g) 180 190 180 140 140 200 180 150 Cycle property (%/100times) 80 80 80 90 90 60 65 65 Electrolyte impregnation property X X X ◯◯ X X ◯ Short-circuit temperature (° C.) 170 170 170 170 170 170 170 170*⁾Proportion of polyolefin resin in total amount of polyolefin resin,inorganic filler and plasticizer

TABLE 4 Example 2- Example 2- Sample Example 2-6 Example 2-7 Example 2-8Example 2-9 10 11 First layer Polypropylene (A) Mv (in units of 10000)40 40 40 40 40 40 Melting point (° C.) 163  163  163  163  163  163  wt% 78 78 56 61 45 53 Propylene-ethylene copolymer Mv (in units of 10000)25 25 25 25 25 25 Melting point (° C.) 160  160  160  160  160  160 Ethylene content (wt %)  3 10  3  3  3  3 wt %  2  2 24  3 19  3High-density polyethylene Mv (in units of 10000) 25 25 25 25 25 25Melting point (° C.) 137  137  137  137  137  137  wt % 20 20 20 16 1614 Silica wt %  0  0  0 20 20 30 Polyolefin resin 35 35 35 30 30 30concentration (wt %)*⁾ Second layer Polypropylene (B) Mv (in units of10000) 40 40 40 40 40 40 Melting point (° C.) 163  163  163  163  163 163  wt %  5  5  5  5  5  5 High-density polyethylene Mv (in units of10000) 25 25 25 25 25 25 Melting point (° C.) 137  137  137  137  137 137  wt %   47.5   47.5   47.5   47.5   47.5   47.5 Mv (in units of10000) 70 70 70 70 70 70 Melting point (° C.) 137  137  137  137  137 137  wt %   47.5   47.5   47.5   47.5   47.5   47.5 Polyolefin resin 3030 30 30 30 30 concentration (wt %)*⁾ Production Layer structure A/B/AA/B/A A/B/A A/B/A A/B/A A/B/A conditions Kneading temperature (° C.)200  200  200  200  200  200  First stretching Stretching temperature (°C.) 123  123  123  123  123  123  Stretching magnification 7 × 7 7 × 7 7× 7 7 × 7 7 × 7 7 × 7 (times) Second stretching Stretching temperature(° C.) 119  119  119  119  119  119  Stretching magnification   1.4  1.4   1.4   1.4   1.4   1.4 (times) Thickness ratio 1/8/1 1/8/1 1/8/11/8/1 1/8/1 1/8/1 Membrane thickness (μm) 18 18 18 18 18 18 Porosity (%)56 55 53 57 57 59 Air permeability (s/100 cc) 240  245  260  210  220 170  Puncture strength (g) 300  300  310  300  300  300  Cycle property(%/100 80 80 80 90 90 90 times) Electrolyte impregnation X X X ◯ ◯ ◯property Short-circuit temperature 200< 200< 200< 200< 200< 200< (° C.)Sample Example 2- Example 2- Example 2- Example 2- ComparativeComparative 12 13 14 15 Example 2-4 Example 2-5 First layerPolypropylene (A) Mv (in units of 10000) 40 40 40 40 40 40 Melting point(° C.) 63 163  163  163  163  163  wt % 47 40 61 40 80 64Propylene-ethylene copolymer Mv (in units of 10000) 25 25 25 25 — —Melting point (° C.) 160  160  160  160  — — Ethylene content (wt %)  3 3  3  3 — — wt %  2  2  3  2  0  0 High-density polyethylene Mv (inunits of 10000) 25 25 25 25 25 25 Melting point (° C.) 137  137  137 137  137  137  wt % 21 18 16 18 20 16 Silica wt % 30 40 20 40  0 20Polyolefin resin 30 30 30 30 35 30 concentration (wt %)*⁾ Second layerPolypropylene (B) Mv (in units of 10000) 40 40 40 40 40 40 Melting point(° C.) 163  163  163  163  163  163  wt %  5  5  5  5  5  5 High-densitypolyethylene Mv (in units of 10000) 25 25 25 25 25 25 Melting point (°C.) 137  137  137  137  137  137  wt %   47.5   47.5   47.5   47.5  47.5   47.5 Mv (in units of 10000) 70 70 70 70 70 70 Melting point (°C.) 137  137  137  137  137  137  wt %   47.5   47.5   47.5   47.5  47.5   47.5 Polyolefin resin 30 30 30 30 30 30 concentration (wt %)*⁾Production Layer structure A/B/A A/B/A B/A/B B/A/B A/B/A A/B/Aconditions Kneading temperature (° C.) 200  200  200  200  200  200 First stretching Stretching temperature (° C.) 123  123  123  123  123 123  Stretching magnification 7 × 7 7 × 7 7 × 7 7 × 7 7 × 7 7 × 7(times) Second stretching Stretching temperature (° C.) 119  119  119 119  119  119  Stretching magnification   1.4   1.4   1.4   1.4   1.4  1.4 (times) Thickness ratio 1/8/1 1/8/1 2/1/2 2/1/2 1/8/1 1/8/1Membrane thickness (μm) 18 18 18 18 18 18 Porosity (%) 59 63 56 63 49 53Air permeability (s/100 cc) 150  120  190  150  420  370  Puncturestrength (g) 320  280  320  260  380  380  Cycle property (%/100 90 9585 90 65 65 times) Electrolyte impregnation ◯ ◯ X X X ◯ propertyShort-circuit temperature 200< 200< 200< 200< 200< 200< (° C.)*⁾Proportion of polyolefin resin in total amount of polyolefin resin,inorganic filler and plasticizer

As is clear from the results of Tables 3 and 4, the microporousmembranes of Embodiment 2 are suitable separators capable of improvingthe cycle property of electricity storage devices.

Embodiment 3

Next, Embodiment 3 is more specifically described with reference toExamples and Comparative Examples; however, Embodiment 3 is not limitedto following Examples unless the gist of Embodiment 3 is exceeded. Thephysical properties in Examples were measured with the followingmethods.

The viscosity average molecular weight (Mv), the membrane thickness, theporosity, the air permeability, the puncture strength and theplasticizer oil absorption amount of the inorganic filler were measuredin the same manner as in Examples of Embodiment 2.

(1) Melting Point (° C.)

The melting point was measured with the DSC 60 manufactured by ShimadzuCorp. A sample (3 mg) was sampled and used as a measurement sample. Thesample was spread over an aluminum open sample pan of 5 mm in diameter,and a cramping cover was placed thereon, and fixed in the pan with asample sealer. The measurement was performed from 30° C. to 200° C. at atemperature increase rate of 10° C./min in a nitrogen atmosphere toobtain a melting endothermic curve. For the obtained melting endothermiccurve, a straight base line was drawn in the range from 85° C. to 175°C., and a heat quantity was calculated from the area of the sectionsurrounded by the straight base line and the melting endothermic curve.This heat quantity was converted into a value per unit mass of thesample to derive the heat of fusion. The temperature corresponding tothe minimum value of the heat of fusion ΔH and the temperaturecorresponding to the minimum of the melting endothermic curve wasmeasured as the melting point.

(2) Primary Particle Size (nm) of Inorganic Particle

A measurement object was sampled from a laminated separator andsubjected to an observation with a scanning electron microscope at amagnification of 30,000 times, and thus the particle sizes of theinorganic particles were identified in a 3.0 μm×3.0 μm field of view.The term “primary particle size” as referred to herein means theparticle size in the condition that the individual particles areindependently dispersed in a matrix, or when the particles areaggregated, the primary particle size means the size of the smallestaggregate particle of the aggregated particles. An average value of theobserved values at ten different positions was taken as the measuredvalue.

(3) Cycle Property (%/100 times)

a. Preparation of Positive Electrode

A slurry was prepared by dispersing 92.2% by mass of a lithium cobaltcomposite oxide (LiCoO₂) as a positive electrode active material, 2.3%by mass of a scale-like graphite and 2.3% by mass of acetylene black,both being conductive, and 3.2% by mass of polyvinylidene fluoride(PVDF) as a binder in N-methylpyrrolidone (NMP). The slurry was appliedonto one side of a 20-μm thick aluminum foil to be a positive electrodecurrent collector with a die coater, dried at 130° C. for 3 minutes, andthen compression molded with a roll press. In this case, the coatingamount of the active material of the positive electrode was regulated tobe 250 g/m² and the active material bulk density of the positiveelectrode was regulated to be 3.00 g/cm³.

b. Preparation of Negative Electrode

A slurry was prepared by dispersing 96.9% by mass of an artificialgraphite as a negative electrode active material, and 1.4% by mass of anammonium salt of carboxymethyl cellulose and 1.7% by mass of astyrene-butadiene copolymer latex, both being binders, in purifiedwater. The slurry was applied onto one side of a 12-μm thick copper foilto be a negative electrode current collector with a die coater, dried at120° C. for 3 minutes, and then compression molded with a roll press. Inthis case, the coating amount of the active material of the negativeelectrode was regulated to be 106 g/m² and the active material bulkdensity of the negative electrode was regulated to be 1.35 g/cm².

c. Preparation of Nonaqueous Electrolyte

A nonaqueous electrolyte was prepared by dissolving LiPF₆ as a solute ina mixed solvent of ethylene carbonate:ethyl methyl carbonate=1:2 (volumeratio) so as for the concentration to be 1.0 mol/L.

d. Assembly of Battery

The separator was cut out as a 30-mm φ disc, and the positive electrodeand the negative electrode were each cut out as a 16-mm φ disc. Thenegative electrode, the separator and the positive electrode weresuperposed in this order so as for the active material sides of thepositive electrode and the negative electrode to face each other, andthe resulting superposed assembly was housed in a stainless steel vesselwith a lid. The vessel and the lid were insulated, and the vessel wasbrought into contact with the copper foil of the negative electrode andthe lid was brought into contact with the aluminum foil of the positiveelectrode. The above-described nonaqueous electrolyte was injected intothe vessel and the vessel was hermetically sealed. The assembled batterywas allowed to stand at room temperature for 1 day, and then the firstcharge of the battery subsequent to the assembly of the battery wasperformed for a total time of 8 hours in such a way that the battery wascharged in an atmosphere of 25° C. with a current value of 2.0 mA (0.33C) until the battery voltage reached 4.2 V; after the attainment of 4.2V, while the battery voltage was being regulated to hold 4.2 V, thecurrent value was decreased starting from 2.0 mA. Successively, thebattery was discharged down to a battery voltage of 3.0 V with a currentvalue of 2.0 mA (0.33 C).

e. Capacity Retention Ratio (%)

In the atmosphere of 60° C., 100 cycles of charge and discharge wereperformed. The charge of the battery was performed for a total time of 3hours in such a way that the battery was charged with a current value of6.0 mA (1.0 C) until the battery voltage reached 4.2 V; after theattainment of 4.2 V, while the battery voltage was being regulated tohold 4.2 V, the current value was decreased starting from 6.0 mA. Thedischarge of the battery was performed with a current value of 6.0 mA(1.0 C) until the battery voltage reached 3.0 V. From the dischargecapacity of the 100th cycle and the discharge capacity of the firstcycle, the capacity retention ratio was calculated. The cases where thecapacity retention ratio was high were evaluated to have a satisfactorycycle property.

(4) Evaluation of High-Temperature Storage Property (%)

The charge of a simple battery assembled as described in theaforementioned a to d was performed in an atmosphere of 25° C., for atotal time of 6 hours in such a way that the battery was charged with acurrent value of 3 mA (about 0.5 C) until the battery voltage reached4.2 V; after the attainment of 4.2 V, while the battery voltage wasbeing regulated to hold 4.2 V, the current value was decreased startingfrom 3 mA. Then the battery was discharged with a current value of 3 mAuntil the battery voltage reached 3.0 V.

Next, the charge of the battery was performed in an atmosphere of 25°C., for a total time of 3 hours in such a way that the battery wascharged with a current value of 6 mA (about 1.0 C) until the batteryvoltage reached 4.2 V; after the attainment of 4.2 V, while the batteryvoltage was being regulated to hold 4.2 V, the current value wasdecreased starting from 6 mA. Then, the battery was discharged with acurrent value of 6 mA until the battery voltage reached 3.0 V. Thedischarge capacity in this case was represented by A (mAh).

Next, the charge of the battery was performed in an atmosphere of 25°C., for a total time of 3 hours in such a way that the battery wascharged with a current value of 6 mA (about 1.0 C) until the batteryvoltage reached 4.2 V; after the attainment of 4.2 V, while the batteryvoltage was being regulated to hold 4.2 V, the current value wasdecreased starting from 6 mA. The battery maintained in a charged statewas allowed to stand in an atmosphere of 60° C. for 7 days. Then, thebattery was taken out from such an atmosphere, and was discharged in anatmosphere of 25° C. with a current value of 6 mA until the batteryvoltage reached 3.0 V. Next, the charge of the battery was performed inan atmosphere of 25° C., for a total time of 3 hours in such a way thatthe battery was charged with a current value of 6 mA (about 1.0 C) untilthe battery voltage reached 4.2 V; after the attainment of 4.2 V, whilethe battery voltage was being regulated to hold 4.2 V, the current valuewas decreased starting from 6 mA. Then, the battery was discharged witha current value of 6 mA until the battery voltage reached 3.0 V. Thedischarge capacity in this case was represented by B (mAh). From theratio of B to A, the capacity retention ratio was calculated as thehigh-temperature storage property.

(5) Shutdown Temperature (° C.) and Short-Circuit Temperature (° C.)

FIG. 1 schematically shows a shutdown temperature measurement apparatus.Reference sign 1 represents a microporous membrane (a laminatedseparator as a measurement object), reference 2A and 2B represent 10-μmthick nickel foils, and reference 3A and 3B represent glass plates.Reference sign 4 represents an electrical resistance measurementapparatus (AG-4311 (trademark), an LCR meter, manufactured by AndoElectric Co., Ltd.) which is connected to the nickel foils 2A and 2B.Reference sigh 5 represents a thermocouple which is connected to athermometer 6. Reference sign 7 represents a data collector which isconnected to the electrical resistance measurement apparatus 4 and thethermometer 6. Reference sign 8 represents an oven which is used forheating the microporous membrane.

More specifically, as shown in FIG. 2, the microporous membrane 1 wassuperposed on the nickel foil 2A, and longitudinally fixed to the nickelfoil 2A with a “Teflon (registered trademark)” tape (the shaded sectionin the figure). The microporous membrane 1 was impregnated with a 1mol/liter lithium borofluoride solution (solvent: propylenecarbonate/ethylene carbonate/γ-butyl lactone=1/1/2). Onto the nickelfoil 2B, as shown in FIG. 3, a “Teflon (registered trademark)” tape (theshaded section in the figure) was bonded, and thus the nickel foil 2Bwas masked with a 15 mm×10 mm window section left unmasked in thecentral section of the foil 2B.

The nickel foil 2A and the nickel foil 2B were superposed on each otherso as to sandwich the microporous membrane 1, and the two-sheets of thenickel foils were sandwiched from the both sides with the glass plates3A and 3B. In this case, the window section of the foil 2B and themicroporous membrane 1 were arranged so as to face each other.

The two sheets of the glass plates were fixed by clipping withcommercially available double clips. The thermocouple 5 was fixed ontothe glass plates with a “Teflon (registered trademark)” tape.

With such an apparatus, the temperature and the electrical resistancewere continuously measured. The temperature was increased from 25° C. to200° C. at a rate of 2° C./min, and the electrical resistance wasmeasured with an alternating current of 1 V and 1 kHz. The shutdowntemperature was defined as the temperature where the electricalresistance of the microporous membrane reached 10³Ω. The short-circuittemperature was defined as the temperature where after the membraneunderwent shutdown and the pores reached a blocked condition, theimpedance again reached a value smaller than 10³Ω.

(6) Content of Copolymerized Monomers

The identification of the comonomers (ethylene and α-olefin) in thepropylene copolymer and the measurement of the content of the comonomerswere performed on the basis of a method on the 13C-NMR method(Macromolecules, 10, 537 (1977)) reported by C. J. Carman et al.

(7) Average Pore Size

As is known, a fluid inside a capillary follows the Poiseuille flow whenthe mean free path of the fluid is smaller than the inner diameter ofthe capillary, and follows the Knudsen flow when the mean free path ofthe fluid is larger than the inner diameter of the capillary. On theassumption that the air flow in the air permeability measurement followsthe Knudsen flow and the water flow in the water permeabilitymeasurement at normal temperature follows the Poiseuille flow, theaverage pore diameter d(m) and the pore tortuosity (τ) (dimensionless)can be obtained from the following formula by using the permeation rateconstant of air R_(gas), the permeation rate constant of water R_(liq),the viscosity of water η (Pa·sec), the standard pressure Ps (101325 Pa),the porosity ε (dimensionless) and the membrane thickness L (m) and themolecular velocity of gas v (m/sec):

d=2v(R _(liq) /R _(gas))(16η/3)(1/Ps)

wherein R_(gas) is obtained from the air permeability (sec) by using thefollowing formula:

R _(gas) (m³/(m²·sec·Pa))=0.0001/(airpermeability)/0.0006424/(0.01276×101325),

and R_(liq) is obtained from the water permeability (cm³/(cm²·sec·Pa))by using the following formula:

R _(liq)(m³/(m²·sec·Pa))=(water permeability)/1000000/0.0001

The water permeability in the foregoing formula is measured as follows:

A microporous membrane which has been immersed in ethanol in advance isset in a stainless steel liquid-permeability cell having a diameter of42 mm, the ethanol remaining on the membrane is washed away with water,and then the membrane is permeated with water at a differential pressureof about 50000 Pa, and the amount of the permeated water (cm³) at anelapsed time of 120 seconds is measured. From the measured amount of thepermeated water, the amount of the permeated water per unit time, unitpressure and unit area is calculated, and the calculated value is takenas the water permeability (to be the water permeability(cm³/(cm²·sec·Pa)).

The molecular velocity of gas v (m/sec) is obtained from the gasconstant R (8.3114 J/mol·K), the absolute temperature T (K), thecircular constant it and the average molecular weight of the air M(=2.896×10⁻²) (kg/mol):

v ²=8RT/πM

Example 3-1

A raw material mixture was prepared by mixing 50.4 parts by mass of apolypropylene (melting point: 163° C.) having a viscosity averagemolecular weight of 400,000, 12.6 parts by mass of a propylene-ethylenerandom copolymer (melting point: 140° C.) having a viscosity averagemolecular weight of 250,000, an ethylene content of 5% by mass and aheat of fusion of 70 J/g, 0.3 part by mass of bis(p-ethylbenzylidene)sorbitol as a nucleating agent, 0.2 part by mass oftetrakis-[methylene-(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionate]methaneas an antioxidant and 63 parts by mass of a paraffin oil (P350P,plasticizer density: 0.868 g/cm³, manufactured by Matsumura Oil ResearchCorp.). The resulting raw material mixture was kneaded with a batch-typemelt kneader (Laboplastomill, manufactured by Toyo Seiki Seisaku-sho,Ltd.) at 200° C. at 50 rpm for 10 minutes. The resulting kneaded mixturewas molded at 5 MPa with a hot press set at 200° C., and heat treated asit was for 3 minutes, and then cooled at 5 MPa with a water-coolingpress controlled at 25° C. to form a 500-μm thick sheet. The sheet wasstretched at 125° C. 5 times×5 times with a simultaneous biaxialstretching machine (manufactured by Toyo Seiki Seisaku-sho, Ltd.), thenthe paraffin oil was extracted and removed with methylene chloride, andthe sheet was dried. The physical properties of the obtainedpolyethylene microporous membrane are shown in Table 5.

Examples 3-2 to 3-8 and Comparative Examples 3-1 to 3-4

Polyethylene microporous membranes were obtained in the same manner asin Example 3-1 except for the conditions shown in Table 5. The physicalproperties of the obtained polyethylene microporous membranes are shownin Table 5.

The oil absorption amount of the silica used as the inorganic filler was200 mL/100 g.

The viscosity average molecular weight and the melting point of thehigh-density polyethylene used were 250,000 and 137° C., respectively.

Example 3-9

The raw material mixture for the surface layer was prepared by mixing50.4 parts by mass of a polypropylene (melting point: 163° C.) having aviscosity average molecular weight of 400,000, 12.6 parts by mass of apropylene-ethylene random copolymer (melting point: 140° C., heat offusion: 70 J/g) having a viscosity average molecular weight of 250,000and an ethylene content of 5% by mass, 0.3 part by mass ofbis(p-ethylbenzylidene) sorbitol as a nucleating agent and 0.2 part bymass of tetrakis-[methylene-(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionate]methane as an antioxidant.

The raw material mixture for the intermediate layer was prepared bymixing 14.25 parts by mass of a high-density polyethylene 1 having aviscosity average molecular weight of 250,000 and a melting point of137° C., 14.25 parts by mass of a high-density polyethylene 2 having aviscosity average molecular weight of 700,000 and a melting point of137° C., 1.5 parts by mass of a polypropylene having a viscosity averagemolecular weight of 400,000 and a melting point of 163° C. and 0.2 partby mass oftetrakis-[methylene-(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionate]methaneas an antioxidant.

The resulting raw material mixtures were fed through feeders to twodouble screw extruders each having a diameter of 25 mm and with L/D=48.Further, 63 parts by mass of a liquid paraffin and 68 parts by mass of aliquid paraffin were added to the raw material mixture for the surfacelayer and the raw material mixture for the intermediate layer,respectively, in such a way that, for each of these raw materialmixtures, liquid paraffin was injected into the concerned double screwextruder by side feeding. The extrusion rates for the both surfacelayers and the intermediate layer were regulated to be 4 kg per 1 hourand 16 kg per 1 hour, respectively. After kneading under the conditionsof 200° C. and 200 rpm, the kneaded raw material mixtures were extrudedfrom a T-die, fixed to the end of each of the extruders, capable ofperforming coextrusion (two-type three-layer) under a condition of 200°C. Immediately, the extruded product was extruded to a roll having asurface temperature controlled at 90° C., and further cooled with a rollhaving a surface temperature controlled at 25° C., and thus, a 1.4-mmthick sheet was formed. The sheet was stretched 7 times×7 times under acondition of 125° C. with a simultaneous biaxial stretching machine,then immersed in methylene chloride to extract and remove the liquidparaffin, and then dried. The dried sheet was stretched with a tenterstretching machine under a condition of 120° C. in the transversedirection with a magnification of 1.5 times. Then, the stretched sheetwas heat treated at 125° C. to be relaxed by 13% in the width direction,and thus a microporous membrane having a two-type three-layer structurein which the two surface layers were the same in composition, and theintermediate layer was different in composition from the surface layers.The physical properties of the obtained microporous membrane are shownin Table 6.

Examples 3-10 to 3-21 and Comparative Examples 3-5 to 3-8

Polyethylene microporous membranes were obtained in the same manner asin Example 3-9 except for the conditions shown in Table 6. The physicalproperties of the obtained polyethylene microporous membranes are shownin Table 6.

Example 3-22

A microporous membrane having a two-type three-layer structure wasobtained in the same manner as in Example 3-9 except that the rawmaterial for the first polyolefin microporous membrane had a compositionincluding 24.4 parts by mass of a polypropylene (melting point: 163° C.)having a viscosity average molecular weight of 400,000, 1.2 parts bymass of a propylene-ethylene copolymer (melting point: 160° C.) having aviscosity average molecular weight of 250,000 and an ethylene content of5% by mass, 5.9 parts by mass of a high-density polyethylene (meltingpoint: 137° C.) having a viscosity average molecular weight of 250,000,0.5 part by mass of a polyethylene (melting point: 120° C.) having aviscosity average molecular weight of 1000 and 8 parts by mass of silicahaving an average primary particle size of 15 nm.

The obtained laminated separator was excellent in the shutdown property.

The oil absorption amount of the silica used as the inorganic filler was200 mL/100 g.

TABLE 5 Sample Example 3-1 Example 3-2 Example 3-3 Example 3-4 Example3-5 Example 3-6 First layer Polypropylene (A) Mv (in units of 10000) 4040 40 40 40 40 Melting point (° C.) 163 163 163 163 163 163 wt % 80 2080 80 64 51 Propylene copolymer Mv (in units of 10000) 25 25 25 25 25 25Melting point (° C.) 140 140 134 140 140 140 Heat of fusion (J/g) 70 7065 70 70 70 Polymerization form Random Random Random Random RandomRandom Content of copolymerized 5 5 15 15 5 5 monomer (wt %) Type ofcopolymerized Ethylene Ethylene Ethylene Ethylene Ethylene Ethylenemonomer wt % 20 80 20 20 16 13 High-density polyethylene Mv (in units of10000) 25 25 25 25 25 25 Melting point (° C.) 137 137 137 137 137 137 wt% 0 0 0 0 20 16 Inorganic filler Type of inorganic filler Silica SilicaSilica Silica Silica Silica Primary particle size of 15 15 15 15 15 15inorganic filler (nm) wt % 0 0 0 0 0 20 Polyolefin resin 37 37 37 37 3737 concentration (wt %)*⁾ Production Layer structure A A A A A Aconditions Kneading temperature (° C.) 200 200 200 200 200 200 Firststretching Stretching temperature 125 125 125 125 125 125 (° C.)Stretching magnification 5 × 5 5 × 5 5 × 5 5 × 5 5 × 5 5 × 5 (times)Second stretching Stretching temperature — — — — — — (° C.) Stretchingmagnification — — — — — — (times) Physical Thickness ratio — — — — — —properties of Membrane thickness (μm) 16 16 16 16 16 16 membranePorosity (%) 54 54 55 55 54 55 Air permeability (s/100 cc) 470 420 480490 380 310 Puncture strength (g) 300 300 360 360 300 300 Average poresize (μm) 0.035 0.037 0.038 0.039 0.04 0.043 Cycle property (%/100 60 6055 55 65 70 times) High-temperature storage 65 66 62 63 64 68 property(%/100 times) Short-circuit temperature 170 170 170 170 170 170 (° C.)Comparative Comparative Comparative Comparative Sample Example 3-7Example 3-8 Example 3-1 Example 3-2 Example 3-3 Example 3-4 First layerPolypropylene (A) Mv (in units of 10000) 40 40 40 40 40 40 Melting point(° C.) 163 163 163 163 163 163 wt % 39 29 90 80 64 90 Propylenecopolymer Mv (in units of 10000) 25 25 25 25 25 25 Melting point (° C.)140 140 140 147 147 147 Heat of fusion (J/g) 70 70 70 70 70 70Polymerization form Random Random Random Random Random Random Content ofcopolymerized 5 5 5 5 5 5 monomer (wt %) Type of copolymerized EthyleneEthylene Ethylene Ethylene Ethylene Ethylene monomer wt % 17 13 10 20 1610 High-density polyethylene Mv (in units of 10000) 25 25 25 25 25 25Melting point (° C.) 137 137 137 137 137 137 wt % 24 18 0 0 0 0Inorganic filler Type of inorganic filler Silica Silica Silica SilicaSilica Silica Primary particle size of 15 15 15 15 15 15 inorganicfiller (nm) wt % 20 40 0 0 20 0 Polyolefin resin 37 37 37 37 37 37concentration (wt %)*⁾ Production Layer structure A A A A A A conditionsKneading temperature (° C.) 200 200 200 200 200 200 First stretchingStretching temperature 125 125 125 125 125 125 (° C.) Stretchingmagnification 5 × 5 5 × 5 5 × 5 5 × 5 5 × 5 5 × 5 (times) Secondstretching Stretching temperature — — — — — — (° C.) Stretchingmagnification — — — — — — (times) Physical Thickness ratio — — — — — —properties of Membrane thickness (μm) 15 17 16 16 17 16 membranePorosity (%) 57 59 53 52 53 51 Air permeability (s/100 cc) 280 160 540520 480 520 Puncture strength (g) 290 310 200 210 240 230 Average poresize (μm) 0.045 0.046 0.031 0.033 0.033 0.033 Cycle property (%/100 7075 45 45 40 45 times) High-temperature storage 66 70 65 65 66 65property (%/100 times) Short-circuit temperature 170 170 170 170 170 170(° C.) *⁾Proportion of polyolefin resin in total amount of polyolefinresin, inorganic filler and plasticizer

TABLE 6 Example Example Example Example Example Example Example ExampleExample Sample 3-9 3-10 3-11 3-12 3-13 3-14 3-15 3-16 3-17 FirstPolypropylene layer Mv (in units of 10000)  40  40  40  40  40  40  40 40  40 (A) Melting point (° C.) 163 163 163 163 163 163 163 163 163 wt%  80  20  80  80  64  51  39  29  39 Propylene copolymer Mv (in unitsof 10000)  25  25  25  25  25  25  25  25  25 Melting point (° C.) 140140 134 140 140 140 140 140 140 Heat of fusion (J/g)  70  70  65  70  70 70  70  70  70 Polymerization form Random Random Random Random RandomRandom Random Random Random Content of copolymerized  5  5  15  15  5  5 5  5  5 monomer (wt %) Type of copolymerized Ethylene Ethylene EthyleneEthylene Ethylene Ethylene Ethylene Ethylene Ethylene monomer wt %  20 80  20  20  16  13  17  13  17 High-density polyethylene Mv (in unitsof 10000)  25  25  25  25  25  25  25  25  25 Melting point (° C.) 137137 137 137 137 137 137 137 137 wt %  0  0  0  0  20  16  24  18  24Inorganic filler Type of inorganic filler Silica Silica Silica SilicaSilica Silica Silica Silica Silica Primary particle size of  15  15  15 15  15  15  15  15  15 inorganic filler (nm) wt %  0  0  0  0  0  20 20  40  20 Polyolefin resin  37  37  37  37  37  37  37  37  37concentration (wt %)*⁾ Second Polypropylene layer Mv (in units of 10000) 40  40  40  40  40  40  40  40  40 (B) Melting point (° C.) 163 163 163163 163 163 163 163 163 wt %  5  5  5  5  5  5  5  5  20 High-densitypolyethylene 1 Mv (in units of 10000)  25  25  25  25  25  25  25  25 25 Melting point (° C.) 137 137 137 137 137 137 137 137 137 wt %  47.5 47.5  47.5  47.5  47.5  47.5  47.5  47.5  40 High-density polyethylene2 Mv (in units of 10000)  70  70  70  70  70  70  70  70  70 Meltingpoint (° C.) 137 137 137 137 137 137 137 137 137 wt %  47.5  47.5  47.5 47.5  47.5  47.5  47.5  47.5  40 Polyolefin resin  32  32  32  32  32 32  32  32  32 concentration (wt %)*⁾ Production Layer structure A/B/AA/B/A A/B/A A/B/A A/B/A A/B/A A/B/A A/B/A A/B/A conditions Kneadingtemperature (° C.) 200 200 200 200 200 200 200 200 200 First stretchingStretching temperature 120 120 120 120 120 120 120 120 120 (° C.)Stretching magnification 7 × 7 7 × 7 7 × 7 7 × 7 7 × 7 7 × 7 7 × 7 7 × 77 × 7 (times) Second stretching Stretching temperature 120 120 120 120120 120 120 120 120 (° C.) Stretching magnification  1.5  1.5  1.5  1.5 1.5  1.5  1.5  1.5  1.5 (times) Physical Thickness ratio 1/8/1 1/8/11/8/1 1/8/1 1/8/1 1/8/1 1/8/1 1/8/1 1/8/1 properties Membrane thickness(μm)  14  14  14  14  14  14  14  14  14 of Porosity (%)  45  46  44  46 48  50  50  52  50 membrane Air permeability (s/100 cc) 400 400 430 410380 240 220 180 220 Puncture strength (g) 300 300 300 300 300 300 300300 300 Average pore size (μm)  0.043  0.048  0.048  0.048  0.048  0.054 0.056  0.058  0.054 Cycle property (%/100  75  75  70  75  75  85  85 90  85 times) High-temperature storage  72  72  70  72  68  77  74  82 74 property (%/100 times) Short-circuit temperature 200< 200< 200< 200<200< 200< 200< 200< 200< (° C.) Comparative Comparative ComparativeComparative Example Example Example Example Example Example ExampleExample Sample 3-18 3-19 3-20 3-21 3-5 3-6 3-7 3-8 First Polypropylenelayer Mv (in units of 10000)  40  40  40  40  40  40  40  40 (A) Meltingpoint (° C.) 163 163 163 163 163 163 163 163 wt %  51  39  39  29  90 80  64  90 Propylene copolymer Mv (in units of 10000)  25  25  25  25 25  25  25  25 Melting point (° C.) 140 140 140 140 140 147 147 147Heat of fusion (J/g)  70  70  70  70  70  70  70  70 Polymerization formRandom Random Random Random Random Random Random Random Content ofcopolymerized  5  5  5  5  5  5  5  5 monomer (wt %) Type ofcopolymerized Ethylene Ethylene Ethylene Ethylene Ethylene EthyleneEthylene Ethylene monomer wt %  13  17  17  13  10  20  16  10High-density polyethylene Mv (in units of 10000)  25  25  25  25  25  25 25  25 Melting point (° C.) 137 137 137 137 137 137 137 137 wt %  16 24  24  18  0  0  0  0 Inorganic filler Type of inorganic filler SilicaSilica Silica Alumina Silica Silica Silica Silica Primary particle sizeof  15  15  15  13  15  15  15  15 inorganic filler (nm) wt %  20  20 20  40  0  0  20  0 Polyolefin resin  37  37  37  37  37  37  37  37concentration (wt %)*⁾ Second Polypropylene layer Mv (in units of 10000) 40  40  40  40  40  40  40  40 (B) Melting point (° C.) 163 163 163 163163 163 163 163 wt %  5  5  20  5  5  5  5  5 High-density polyethylene1 Mv (in units of 10000)  25  25  25  25  25  25  25  25 Melting point(° C.) 137 137 137 137 137 137 137 137 wt %  47.5  47.5  40  47.5  47.5 47.5  47.5  47.5 High-density polyethylene 2 Mv (in units of 10000)  70 70  70  70  70  70  70  70 Melting point (° C.) 137 137 137 137 137 137137 137 wt %  47.5  40  40  47.5  47.5  47.5  47.5  47.5 Polyolefinresin  32  32  32  32  32  32  32  32 concentration (wt %)*⁾ ProductionLayer structure B/A/B B/A/B B/A/B A/B/A A/B/A A/B/A A/B/A A/B/Aconditions Kneading temperature (° C.) 200 200 200 200 200 200 200 200First stretching Stretching temperature 120 120 120 120 120 120 120 120(° C.) Stretching magnification 7 × 7 7 × 7 7 × 7 7 × 7 7 × 7 7 × 7 7 ×7 7 × 7 (times) Second stretching Stretching temperature 120 120 120 120120 120 120 120 (° C.) Stretching magnification  1.5  1.5  1.5  1.5  1.5 1.5  1.5  1.5 (times) Physical Thickness ratio 2/1/2 2/1/2 2/1/2 1/8/11/8/1 1/8/1 1/8/1 1/8/1 properties Membrane thickness (μm)  14  14  14 14  14  14  14  14 of Porosity (%)  49  50  50  51  45  46  50  44membrane Air permeability (s/100 cc) 240 200 200 190 480 530 480 500Puncture strength (g) 300 310 300 310 300 300 300 300 Average pore size(μm)  0.055  0.057  0.055  0.058  0.036  0.038  0.04  0.036 Cycleproperty (%/100  85  85  85  90  60  55  60  55 times) High-temperaturestorage  76  72  68  83  72  72  73  72 property (%/100 times)Short-circuit temperature 200< 200< 200< 200< 200< 200< 200< 200< (° C.)*⁾Proportion of polyolefin resin in total amount of polyolefin resin,inorganic filler and plasticizer

As is clear from the results of Tables 5 and 6, the microporousmembranes of Embodiment 3 are suitable separators capable of improvingthe cycle property of electricity storage devices.

Embodiment 4

Next, Embodiment 4 is more specifically described with reference toExamples and Comparative Examples; however, Embodiment 4 is not limitedto following Examples unless the gist of Embodiment 4 is exceeded. Thephysical properties in Examples were measured with the followingmethods.

The following properties were measured in the same manners as inExamples of foregoing Embodiments 2 and 3: the viscosity averagemolecular weight (Mv), the melting point, the primary particle size ofthe inorganic particle, the membrane thickness, the porosity, the airpermeability, the puncture strength, the cycle property, the evaluationof the high-temperature storage property, the shutdown temperature, theshort-circuit temperature, the content of the copolymerized monomer, andthe plasticizer oil absorption amount of the inorganic filler.

Example 4-1

A raw material mixture was prepared by mixing 12.9 parts by mass of apolypropylene (melting point: 163° C.) having a viscosity averagemolecular weight of 400,000, 5.7 parts by mass of a propylene-ethylenerandom copolymer (melting point: 140° C.) having a viscosity averagemolecular weight of 250,000 and an ethylene content of 5% by mass, 8.0parts by mass of a high-density polyethylene (melting point: 137° C.)having a viscosity average molecular weight of 250,000, 8 parts by massof silica (oil absorption amount: 200 ml/100 g), 0.3 part by mass ofbis(p-ethylbenzylidene) sorbitol as a nucleating agent, 0.2 part by massof tetrakis-[methylene-(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionate]methane as an antioxidant and 62 parts by mass of a paraffinoil (P350P, plasticizer density: 0.868 g/cm³, manufactured by MatsumuraOil Research Corp.). The resulting raw material mixture was kneaded witha batch-type melt kneader (Laboplastomill, manufactured by Toyo SeikiSeisaku-sho, Ltd.) at 200° C. at 50 rpm for 10 minutes. The resultingkneaded mixture was molded at 5 MPa with a hot press set at 200° C., andheat treated as it was for 3 minutes, and then cooled at 5 MPa with awater-cooling press controlled at 25° C. to form a 500-μm thick sheet.The sheet was stretched at 125° C. 5 times×5 times with a simultaneousbiaxial stretching machine (manufactured by Toyo Seiki Seisaku-sho,Ltd.), then the paraffin oil was extracted and removed from the sheetwith methylene chloride, and the sheet was dried. The physicalproperties of the obtained polyethylene microporous membrane are shownin Table 7.

Examples 4-2 to 4-6 and Comparative Examples 4-1 to 4-3

Polyethylene microporous membranes were obtained in the same manner asin Example 4-1 except for the conditions shown in Table 7. The physicalproperties of the obtained polyethylene microporous membranes are shownin Table 7.

Example 4-7

The raw material mixture for the surface layer was prepared by stirringwith a mixer 16.3 parts by mass of a polypropylene (melting point: 163°C.) having a viscosity average molecular weight of 400,000, 7.2 parts bymass of a propylene-ethylene random copolymer (melting point: 140° C.,heat of fusion: 70 J/g) having a viscosity average molecular weight of250,000 and an ethylene content of 5% by mass, 10.1 parts by mass of ahigh-density polyethylene (melting point: 137° C.) having a viscosityaverage molecular weight of 250,000, 14.4 parts by mass of silica (oilabsorption amount: 200 ml/100 g), 0.3 part by mass ofbis(p-ethylbenzylidene) sorbitol as a nucleating agent, 0.2 part by massof tetrakis-[methylene-(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionate]methane as an antioxidant and 17.3 parts by mass of a liquidparaffin (P350P, manufactured by Matsumura Oil Research Corp.) as aplasticizer.

The raw material mixture for the intermediate layer was prepared bymixing 14.25 parts by mass of a high-density polyethylene 1 having aviscosity average molecular weight of 250,000 and a melting point of137° C., 14.25 parts by mass of a high-density polyethylene 2 having aviscosity average molecular weight of 700,000 and a melting point of137° C., 1.5 parts by mass of a polypropylene having a viscosity averagemolecular weight of 400,000 and a melting point of 163° C. and 0.2 partby mass oftetrakis-[methylene-(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionate]methaneas an antioxidant.

The resulting raw material mixtures were fed through feeders to twodouble screw extruders each having a diameter of 25 mm and with L/D=48.Further, 60 parts by mass of a liquid paraffin and 70 parts by mass of aliquid paraffin were added to the raw material mixture for the surfacelayer and the raw material mixture for the intermediate layer,respectively, in such a way that, for each of these raw materialmixtures, liquid paraffin was injected into the concerned double screwextruder by side feeding. The extrusion rates for the both surfacelayers and the intermediate layer were regulated to be 4 kg per 1 hourand 16 kg per 1 hour, respectively. After kneading under the conditionsof 200° C. and 200 rpm, the kneaded raw material mixtures were extrudedfrom a T-die, fixed to the end of each of the extruders, capable ofperforming coextrusion (two-type three-layer) under a condition of 220°C. Immediately, the extruded product was extruded to a roll having asurface temperature controlled at 70° C., and further cooled with a rollhaving a surface temperature controlled at 25° C., and thus, a 1.4-mmthick sheet was formed. The sheet was stretched 7 times×7 times under acondition of 125° C. with a simultaneous biaxial stretching machine,then immersed in methylene chloride to extract and remove the liquidparaffin, and then dried. The dried sheet was stretched with a tenterstretching machine under a condition of 125° C. in the transversedirection with a magnification of 1.5 times. Then, the stretched sheetwas heat treated at 132° C. to be relaxed by 21% in the width direction,and thus a microporous membrane having a two-type three-layer structurein which the two surface layers were the same in composition, and theintermediate layer was different in composition from the surface layers.The physical properties of the obtained microporous membrane are shownin Table 8.

Examples 4-8 to 4-17 and Comparative Examples 4-4 to 4-6

Polyethylene microporous membranes were obtained in the same manner asin Example 4-7 except for the conditions shown in Table 8. The physicalproperties of the obtained polyethylene microporous membranes are shownin Table 8.

Example 4-18

A microporous membrane having a two-type three-layer structure wasobtained in the same manner as in Example 4-7 except that the rawmaterial for the first polyolefin microporous membrane had a compositionincluding 24.4 parts by mass of a polypropylene (melting point: 163° C.)having a viscosity average molecular weight of 400,000, 1.2 parts bymass of a propylene-ethylene copolymer (melting point: 160° C.) having aviscosity average molecular weight of 250,000 and an ethylene content of5% by mass, 5.9 parts by mass of a high-density polyethylene (meltingpoint: 137° C.) having a viscosity average molecular weight of 250,000,0.5 part by mass of a polyethylene (melting point: 120° C.) having aviscosity average molecular weight of 1000 and 8 parts by mass of silicahaving an average primary particle size of 15 nm.

The obtained laminated separator was excellent in the shutdown property.

TABLE 7 Sample Example 4-1 Example 4-2 Example 4-3 Example 4-4 Example4-5 First layer (A) Polypropylene Mv (in units of 10000) 40 40 40 40 40Melting point (° C.) 163 163 163 163 163 wt % 34 15 34 34 34 Propylenecopolymer Mv (in units of 10000) 25 25 25 25 25 Melting point (° C.) 140140 125 140 140 Heat of fusion (J/g) 70 70 70 40 70 Polymerization formRandom Random Random Random Random Type of copolymerized monomerEthylene Ethylene Ethylene Ethylene Ethylene Content of copolymerized 55 15 5 5 monomer (wt %) wt % 15 34 15 15 8 High-density polyethylene Mv(in units of 10000) 25 25 25 25 25 Melting point (° C.) 137 137 137 137137 wt % 21 21 21 21 28 Inorganic filler Type of inorganic filler SilicaSilica Silica Silica Silica Primary particle size of inorganic 15 15 1515 15 filler (nm) wt % 30 30 30 30 30 Propylene copolymer/inorganic 0.51.1 0.5 0.5 0.3 filler (mass ratio) Polyolefin resin concentration 38 3838 38 38 (wt %)*⁾ Second layer Polypropylene (B) Mv (in units of 10000)Melting point (° C.) wt % High-density polyethylene Mv (in units of10000) Melting point (° C.) wt % Mv (in units of 10000) Melting point (°C.) wt % Polyolefin resin concentration (wt %)*⁾ Production Layerstructure A A A A A conditions Kneading temperature (° C.) 200 200 200200 200 First stretching Stretching temperature (° C.) 125 125 125 125125 Stretching magnification (times) 5 × 5 5 × 5 5 × 5 5 × 5 5 × 5Second stretching Stretching temperature (° C.) — — — — — Stretchingmagnification (times) — — — — — Physical Thickness ratio — — — — —properties of Membrane thickness (μm) 18 18 16 18 19 membrane Porosity(%) 52 52 51 52 53 Air permeability (s/100 cc) 350 380 380 340 300Puncture strength (g) 300 300 320 310 300 Cycle property (%/100 times)70 70 70 70 75 High-temperature storage property 66 64 63 61 60 (%/100times) Short-circuit temperature (° C.) 170 170 170 170 170 ComparativeComparative Comparative Sample Example 4-6 Example 4-1 Example 4-2Example 4-3 First layer (A) Polypropylene Mv (in units of 10000) 40 4040 40 Melting point (° C.) 163 163 163 163 wt % 29 34 32 34 Propylenecopolymer Mv (in units of 10000) 25 25 25 25 Melting point (° C.) 140152 140 105 Heat of fusion (J/g) 70 70 70 70 Polymerization form RandomRandom Random Random Type of copolymerized monomer Ethylene EthyleneEthylene Ethylene Content of copolymerized 15 3 5 30 monomer (wt %) wt %13 15 32 15 High-density polyethylene Mv (in units of 10000) 25 25 25 25Melting point (° C.) 137 137 137 137 wt % 18 30 16 30 Inorganic fillerType of inorganic filler Silica Silica Silica Silica Primary particlesize of inorganic 15 15 15 15 filler (nm) wt % 40 30 20 30 Propylenecopolymer/inorganic 0.3 0.5 1.6 0.5 filler (mass ratio) Polyolefin resinconcentration 38 38 38 38 (wt %)*⁾ Second layer Polypropylene (B) Mv (inunits of 10000) Melting point (° C.) wt % High-density polyethylene Mv(in units of 10000) Melting point (° C.) wt % Mv (in units of 10000)Melting point (° C.) wt % Polyolefin resin concentration (wt %)*⁾Production Layer structure A A A A conditions Kneading temperature (°C.) 200 200 200 200 First stretching Stretching temperature (° C.) 125125 125 125 Stretching magnification (times) 5 × 5 5 × 5 5 × 5 5 × 5Second stretching Stretching temperature (° C.) — — — — Stretchingmagnification (times) — — — — Physical Thickness ratio — — — —properties of Membrane thickness (μm) 19 18 17 18 membrane Porosity (%)55 51 52 52 Air permeability (s/100 cc) 230 370 950 1200 Puncturestrength (g) 340 320 300 330 Cycle property (%/100 times) 80 65 40 30High-temperature storage property 70 65 55 50 (%/100 times)Short-circuit temperature (° C.) 170 170 170 170 *⁾Proportion ofpolyolefin resin in total amount of polyolefin resin, inorganic fillerand plasticizer

TABLE 8 Example Example Example Example Example Example Example ExampleExample Example Example Comparative Comparative Comparative Sample 4-74-8 4-9 4-10 4-11 4-12 4-13 4-14 4-15 4-16 4-17 Example 4-4 Example 4-5Example 4-6 First layer Polypropylene (A) Mv (in units of 10000)  40  40 40  40  40  40  40  40  40  40  40  40  40  40 Melting point (° C.) 163163 163 163 163 163 163 163 163 163 163 163 163 163 wt %  34  15  34  34 34  29  34  8  15  25  29  34  32  34 Propylene copolymer Mv (in unitsof 10000)  25  25  25  25  25  25  25  25  25  25  25  25  25  25Melting point (° C.) 140 140 125 140 140 140 140 140 140 140 140 152 140105 Heat of fusion (J/g)  70  70  70  40  70  70  70  70  70  70  70  70 70  70 Polymerization form Random Random Random Random Random RandomRandom Random Random Random Random Random Random Random Type ofcopolymerized Ethylene Ethylene Ethylene Ethylene Ethylene EthyleneEthylene Ethylene Ethylene Ethylene Ethylene Ethylene Ethylene Ethylenemonomer Content of copolymerized  5  5  15  5  5  15  5  5  5  5  15  3 5  30 monomer (wt %) wt %  15  34  15  15  8  13  15  15  34  10  13 15  32  15 High-density polyethylene Mv (in units of 10000)  25  25  25 25  25  25  25  25  25  25  25  25  25  25 Melting point (° C.) 137 137137 137 137 137 137 137 137 137 137 137 137 137 wt %  21  21  21  21  28 18  21  42  21  15  18  30  16  30 Inorganic filler Type of inorganicfiller Silica Silica Silica Silica Silica Silica Silica Silica SilicaSilica Alumina Silica Silica Silica Primary particle size of  15  15  15 15  15  15  15  15  15  15  13  15  15  15 inorganic filler (nm) wt % 30  30  30  30  30  40  30  30  30  50  40  30  20  30 Propylene  0.5 1.1  0.5  0.5  0.3  0.3  0.5  0.5  1.1  0.2  0.3  0.5  1.6  0.5copolymer/inorganic filler (mass ratio) Polyolefin resin  38  38  38  38 38  38  38  38  38  38  38  38  38  38 concentration (wt %)*⁾ SecondPolypropylene layer (B) Mv (in units of 10000)  40  40  40  40  40  40 40  40  40  40  40  40  40  40 Melting point (° C.) 163 163 163 163 163163 163 163 163 163 163 163 163 163 wt %  5  5  5  5  5  5  5  20  5  5 5  5  5  5 High-density polyethylene 1 Mv (in units of 10000)  25  25 25  25  25  25  25  25  25  25  25  25  25  25 Melting point (° C.) 137137 137 137 137 137 137 137 137 137 137 137 137 137 wt %  47.5  47.5 47.5  47.5  47.5  47.5  47.5  40  47.5  47.5  47.5  47.5  47.5  47.5High-density polyethylene 2 Mv (in units of 10000)  70  70  70  70  70 70  70  70  70  70  70  70  70  70 Melting point (° C.) 137 137 137 137137 137 137 137 137 137 137 137 137 137 wt %  47.5  47.5  47.5  47.5 47.5  47.5  47.5  40  47.5  47.5  47.5  47.5  47.5  47.5 Polyolefinresin 32 32 32 32 32 32 32 32 32 32 32 32 32 32 concentration (wt %)*⁾Production Layer structure A/B/A A/B/A A/B/A A/B/A A/B/A A/B/A B/A/BA/B/A B/A/B A/B/A A/B/A A/B/A A/B/A A/B/A conditions Kneadingtemperature (° C.) 200 200 200 200 200 200 200 200 200 200 200 200 200200 First stretching Stretching temperature (° C.) 125 125 125 125 125125 125 125 125 125 125 125 125 125 Stretching magnification 7 × 7 7 × 77 × 7 7 × 7 7 × 7 7 × 7 7 × 7 7 × 7 7 × 7 7 × 7 7 × 7 7 × 7 7 × 7 7 × 7(times) Second stretching Stretching temperature (° C.) 132 132 132 132132 132 132 132 132 132 132 132 132 132 Stretching magnification  1.5 1.5  1.5  1.5  1.5  1.5  1.5  1.5  1.5  1.5  1.5  1.5  1.5  1.5 (times)Physical Thickness ratio 1/8/1 1/8/1 1/8/1 1/8/1 1/8/1 1/8/1 2/1/2 1/8/12/1/2 1/8/1 1/8/1 1/8/1 1/8/1 1/8/1 properties Membrane thickness (μm) 14  14  13  13  14  13  14  14  14  14  14  14  14  14 of Porosity (%) 40  40  41  38  41  43  40  44  40  46  44  42  43  42 membrane Airpermeability (s/100 cc) 210 230 240 260 190 180 180 170 200 150 200 260880 900 Puncture strength (g) 300 300 320 310 300 340 300 300 300 300320 320 300 330 Cycle property (%/100  90  85  80  75  80  95  90  90 85  95  90  65  50  45 times) High-temperature storage  83  81  78  81 75  82  80  68  79  85  83  83  68  64 property (%/100 times)Short-circuit temperature 200< 200< 200< 200< 200< 200< 200< 200< 200<200< 200< 200< 200< 200< (° C.) *⁾Proportion of polyolefin resin intotal amount of polyolefin resin, inorganic filler and plasticizer

As is clear from the results of Tables 7 and 8, the microporousmembranes of Embodiment 4 are suitable separators capable of improvingthe cycle property of electricity storage devices.

The present application is based on Japanese Patent Application(Japanese Patent Application No. 2009-054795) filed at Japan PatentOffice on Mar. 9, 2009, Japanese Patent Application (Japanese PatentApplication No. 2009-064533) filed at Japan Patent Office on Mar. 17,2009, Japanese Patent Application (Japanese Patent Application No.2009-280486) filed at Japan Patent Office on Dec. 10, 2009, JapanesePatent Application (Japanese Patent Application No. 2009-280488) filedat Japan Patent Office on Dec. 10, 2009, Japanese Patent Application(Japanese Patent Application No. 2010-021859) filed at Japan PatentOffice on Feb. 3, 2010, and Japanese Patent Application (Japanese PatentApplication No. 2010-022481) filed at Japan Patent Office on Feb. 3,2010, and the contents thereof are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

According to the present invention, a separator having a high level ofcompatibility between the heat resistance, the cycle property and theshutdown property is provided. Such a separator has an industrialapplicability, in particular as separators for lithium ion batteries.

Also, according to the present invention, a polyolefin microporousmembrane suitable a separator capable of improving the cycle property ofelectricity storage devices is provided.

REFERENCE SIGNS LIST

-   -   1: Microporous membrane    -   2A, 2B: Nickel foil of 10 μm in thickness    -   3A, 3B; Glass plate    -   4: Electrical resistance measurement apparatus    -   5: Thermocouple    -   6: Thermometer    -   7: Data collector    -   8: Oven

1-3. (canceled)
 4. A laminated polyolefin microporous membranecomprising 50 to 99% by mass of polypropylene and 1 to 50% by mass ofpropylene-α-olefin copolymer, wherein a content of the α-olefin in thepropylene-α-olefin copolymer is more than 1% by mass and 15% by mass orless, and at least one side of the laminated polyolefin microporousmembrane is laminated.
 5. The laminated polyolefin microporous membraneaccording to claim 4, wherein a mixing ratio(polypropylene/propylene-α-olefin copolymer) (mass ratio) between thepolypropylene and the propylene-α-olefin copolymer is 1.5 or more and 60or less. 6-10. (canceled)
 11. A method for producing the laminatedpolyolefin microporous membrane according to claim 4, the methodcomprising: (1) a kneading step of forming a kneaded mixture by kneadinga polyolefin resin comprising 50 to 99% by mass of polypropylene and 1to 50% by mass of a propylene copolymer, and a plasticizer; (2) a sheetmolding step, following the kneading step, of processing the kneadedmixture into a sheet-like molded body by extruding the kneaded mixtureinto a sheet shape and by cooling and solidifying the sheet-shapedproduct; (3) a stretching step, following the molding step, of forming astretched product by biaxially stretching the sheet-like molded bodywith an area magnification of 20× or more and 200× or less; (4) a porousbody forming step, following the stretching step, of forming a porousbody by extracting the plasticizer from the stretched product; (5) aheat treatment step, following the porous body forming step, of heattreating the porous body at a temperature equal to or lower than amelting point of the polyolefin resin and widthwise stretching theporous body to produce a polyolefin microporous membrane; and (6) alaminating step of laminating at least one side of the polyolefinmicroporous membrane, wherein the content of the α-olefin in thepropylene-α-olefin copolymer is more than 1% by mass and 15% by mass orless. 12-13. (canceled)
 14. The method according to claim 11, wherein aheat of fusion of the propylene copolymer is 60 J/g or more. 15.(canceled)
 16. The laminated polyolefin microporous membrane of claim 4,wherein the polyolefin microporous membrane consists of thepolypropylene and the propylene-α-olefin copolymer.